• Being smaller, then need less food, less housing, etc. So they have a lower cost of living per person, and can live on a lower wage. Have them look for jobs which call for a single mind, no matter how small the body and hands attached to that mind. Have them work as clerks.
• On a related note, they can either write on much smaller parchments (saving money) or produce beautiful illuminated manuscripts.
• They might be able to find jobs as goldsmiths and jewelers, too. (Or, as John pointed out in the Comments, other delicate things like cloth.)
• Or they get employed as low-end security guards. Their job is to stand next to the coin chest and shout out loud if anything happens. They would not be expected to stop a human burglar, just raise the alert. (See cost-of-living and wages, above, for why they replace humans in this role.)

• What do mainstream religions think about them? In the middle ages, the poor could earn room and board by praying for the salvation of the rich.

1. cleaners, their small size and ability to climb would make them excellent at cleaning in hard to reach places like corners ceilings, and under furniture.

2. plumbers, they can fit down a common drain to unclog it, their small hands would be excellent for designing fountain water systems.

3. tinker's/toymakers they would be able to manipulate small delicate wires and springs to make complex toys and gadgets.

4. artists, they would be able to paint small details and sculpt intricate designs.

Aviation.

They're bigger than bats, smaller than turkeys. No, they're not evolved to fly. But in a world capable of making wagons, they ought to be able to flap the wings or turn the rotors of some fantasy gecko-powered aircraft. (Their own little Leonardo da Vinci probably designed it) If I assume your fantasy setting is short on jet planes and drones, then they are the only option that can reconnoiter for your caravan journeys and spot the ambushes literally a mile away. Their small planes can glide quietly into hostile terrain at night - no matter how small they are, they can still set hundreds of brush fires behind enemy lines, sneak into command tents to look at maps and listen to plans, or use some of that fancy magic you mentioned to dump a near-bottomless sack of cockatrice dandruff into the enemy's next batch of field rations.

1. Watchmakers. A pocket watch is a pre-industrial invention (made by humans with excellent vision and magnifying glasses), and the geckos, if they were not cut short in their eyesight, make excellent watchmakers. They consistently outperform everyone else, gain a complete monopoly and keep high prices. A watch is a delicate mechanism taking a long time to make and quite an expensive one, but everyone even slightly rich needs it to prove his/her/their status.

2. Doctors. They are good in suturing wounds, their biology being different from human means there is no risk of infection (in both directions). They are not that good in anything else, though perhaps they see infrared or are polychromats (some reptiles are tetrachromats, so there is a possibility) and can recognize medical conditions based on skin tone (like in Egan's Seventh Sight) and by time, they intuitively develop some common remedies (and some wrong ones). Given how widespread was the use of leeches for bloodletting in our history, the bar is low for the geckos to earn a long lasting fame as state-of-the-art healers.

Vermin control

Mice, rats, rabbits and insects do vast amounts of damage to crops and/or stored food. Humans have limited ability to target them efficiently, but sapient geckos can take the fight to them, using their intelligence and human-provided metal tools* to eliminate them in places that humans cannot reach. Giving even a 1% share of a farms crops or food stores to the sapient geckos is a win-win situation for both the farm and a very large number of geckos. (Note that a bounty on vermin killed is not a good incentive scheme - it creates a perverse incentive for the geckos to hunt a large number of vermin who may have been nowhere near the food while ignoring the larger creatures like rabbits that are too large for the geckos to drag in for the bounty. The geckos can still have a secondary income stream from furs taken from their kills.)

Many of the creatures they will be targeting will be larger than the geckos and a significant threat. However, like primitive humans hunting mammoths, intelligence and weapons will let the geckos prevail.

*The humans need to provide the metal tools since the ability to work metal does not scale well with small size.

They're excellent diplomats and messengers. They can be transported almost as fast as a regular mail, yet they can use their own judgements and sender's instructions to decide which message to unveil and when, ie, revealing a sensitive message after a correct countersign, or divulging a potentially divisive policy only if the situation call for it.

If they're as intelligent as average human and can live for decades, they can be teachers and instructors, maybe even scholars. Being cheaper to hire is the obvious advantage, and if they can read and write on itty bitty books, they will also speed up the general dissemination of knowledge.

Since you're preindustrial, look into lacemaking and fine embroidery. Both of these are high return for relatively small scale work requiring fine detail that could be relatively large detail to your smaller people. On larger scales both would bring in significant amounts of money.

Lacemaking especially was a cottage industry and entire villages would work together to produce larger items with individuals creating specialised items and others stitching them together into the full piece.

five jobs for tiny gecko people that haven't been mentioned yet:

1. beekeepers: being small allows them to move among hives without disturbing bees as much as humans. they can harvest honey, maintain hives, and detect early signs of colony problems - acting as 'bee administrators.' honey and beeswax were valuable commodities in pre-industrial societies. they just have to wear intense protective suits while spelunking through the beehives.

2. cartographers/surveyors of small spaces: they can map cave systems, sewers, within houses, and other confined spaces that humans can't easily access. accurate maps of these areas would be extremely valuable to cities, mines, and military operations.

3. musical instrument repairers: they can reach inside lutes, harps, and other complex instruments to tune and repair internal mechanisms that human hands can't access without disassembling the instrument. wealthy musicians would pay premium prices for this service.

4. glasswork detailers: they could add intricate designs and details to blown glass that human hands find impossible to create. their tiny hands could manipulate molten glass or etching tools with unprecedented precision, creating luxury goods for wealthy patrons.

5. manuscript miniaturists: they could create incredibly detailed religious texts and illustrations at scales impossible for humans, producing entire books that fit in a locket or prayer beads. these'd be luxury religious items for nobility and clergy.

Chimney sweeps

Up until the 19th century, chimney sweeps would employ children to climb up the flue to clean it. If these people also get the gecko's natural climbing ability, they could do the work more easily. Plus, with their smaller size and climbing ability they are less likely to get stuck, which would require a costly breaking and repairing of the chimney.

Some other ideas:

• Artist, but in the sense of performative arts: during long travels, nobles or rich merchants could bring with themselves a portable theatre company who could enact some of the most popular plays in a small box-like theatre (they could also go to the nearby pub to hear the local gossip and have one of them tell the news during the pause between the acts...)
• Sailors: they could replace some of the crew on the less demanding tasks (cartography, small repairs), reducing a bit the need for supplies on the ships (but probably this would be a negligible improvement, since probably they couldn't replace more than a pair of men)
• Help to travelers: if somebody had to cross a forest alone, it could be possible to bring 2 or 3 of them in the backpack. They wouldn't need a lot of food, but could help by mounting guard during the night or being sent in vanguard to detect possible dangers

They would be just as intellectually capable as humans. Their size wouldn’t matter as long as they can use their brains. I think the smartest of them could easily match human scientists, and maybe even surpass them, because they would be able to perfect the smaller components of experiments. They would also be in high demand as mechanics. Not a lot of complex machinery was made in the pre-industrial age, but it existed. Clockmakers and toymakers would be the best professions for them. They would be able to fine tune every gear and every component. They could also be surgeons. Complex machines didn’t exist at the time, so there wasn’t any machinery to help humans with accuracy. Surgeries that are easily performed with machinery today would’ve been impossible in the pre-industrial age. The gecko people, being small, would be able to perform the finer parts of operations that humans aren’t precise enough to do.

This has been explored in numerous fictional franchises before, including Terry Pratchett's Discworld, where they basically fitted into every niche where a small size was a benefit and their low resource consumption (They need less food, and smaller housing, etc) allowed them to undercut everyone else.

Wee Mad Arthur the rat catcher was a notable example as he could follow rats in to their tunnels to lay traps or put down poison.

They existed in between the gaps left by humans.

Ironically, that franchise also had trolls, whom considered humans to be the equivalent of a little person. And Dwarfs who were in between humans an the smallest races.

Another was the Anime Beastars, where a society of mixed size anthropomorphised animal exists, and smaller creatures fulfill most common functions in society where a large size isn't necessary.

For example there are no size restrictions for being an accountant.

Disneys Zootopia depicts an entire society of smaller creatures with their own economy, performing all of the normal tasks for their own sized creatures.

As others have pointed out, due to their small size they need less money for a living, less food, less living space and so on.
Therefore I expect them to have a lower wage and thus I would expect them to be used for all kind of purposes for which you want cheap employees which don't need to do heavy physical work.

I would expect them as domestic workers like cooks, house cleaners and so on. Probably just hire multiple to get the job done.
I am not sure if they would be a good fit for a nanny. "No, this is your nanny and not a toy".

Also shoeblacks, sales assistants and messengers seem like fitting jobs.

Actually when they are so much weaker than humans, I fear humans would be tempted to use them as bondservants or slaves as the geckos are pretty useful and have limited possibilities to defend against us. I think in the direction of how the elves are treated in Harry Potter. Fortunately you can choose how the society will be and if there is a fair treaty between humans and geckos.

Inspector. They can get into things to check them out far better than a human can, especially since this is before the days of robotics. And providing inspection access to a gecko is far easier than providing inspection access to a human.

Some of them: Music boxes, Entertainment

Basically like a gramophone. Your geckos, sometimes a small choir of them, live in music boxes and sing or play instruments. It's a big, bulky box, easily big enough for living quarters, and can be found in taverns. Customers put a coin in, and the geckos start playing. The sound is amplified, or focussed, by a big tube, like a megaphone.

It has lots of advantages over a human band. Small, doesn't drink the bar dry, needs less money. It's also got an advantage over a mechanical jukebox, in that it can throw peanuts or trash at the jerk who has put that song on for the third time in a row.

You can also build them a little stage, and they can act out plays for you, or sports matches..

They’re geckos, so they can climb anything. They’ll be in demand as roofers, despite their small size. Good as chimney sweeps, replacing the need to send small children up chimneys. They’d also make fantastic cat burglars.

Office work, engineering

People with essentially any limitations can do a work at an office. Or work on designing things. Or be an assistant, secretary, archivist, librarian, … Unless the creatures have low intelligence, it is possible.

However, there would be some limitations caused by the size: Sizes (and weights!) of documents in offices can be too large to allow safe handling of them. But I think that engineering work could be done – even if they would work with documents of sizes more appropriate for their body sizes. But the document sizes could make it harder for others to read in them. The tools, such as pencils, drafting tables etc. could be manufactured in the appropriate sizes for them without much problem.

Consultants

There are even some kinds of work, where the interaction with others is done essentially only by talking. If your world allow this, they could provide consultation services for magic (or even directly provide magic-involving services?), psychology, they could work also as mathematicians, scientists or philosophers.

Well, being psychologist is relatively new job. Assuming pre-industrial world, I'm quite unsure whether the psychology would even exist.

I can imagine them as scientists, secretaries or engineers quite easily.

You have the opposite problem

They will economically crush humans.

They are even better than humans for muscle power! The efficiency of calories to mechanical work does not change much between animals and is 20-25% or so.

But the power of an animal scales as mass^(3/4), which means that a human's mass worth of 15cm-tall geko-people can output 6 or 7 times the muscle power of a human assuming human proportions.

So even just considering using them to drive mills, they would have an edge in terms of making the engine more compact.

But the real advantage, as mentioned in the other answers, is in intellectual work and the skilled trades. Being smaller allows precise artwork and machining and may accelerate the development of steam power (which requires tight tolerances on machine parts).

For these tasks, which are delicate instead of simply a matter of strength and power, the advantage is primarily in the much lower cost of living (mainly food in the past, housing today).

If they can reproduce, humans would go extinct. Sure, humans are stronger and can kill them in a riot. But they have the numbers and can organize. Stone age technology was enough to kill off mammoths. And we can't even get rid of low-intelligence pests such as cockroaches.

If they must be magically created, their creators would rule. The tricky thing is that the creators cannot magically control them. But they can still enslave them in cages etc to do work. The muggles have to room and board one person per slave they own. These wizards have a buy 1 get 50 free deal.

An important thing to keep in mind is that the life expectancy for a gecko in a predator-free environment is only around 15 years. Subtract the time it takes them to reach maturity and the time when they're too old to do much work and you're left with 11-12 years for a career.

Also, geckos are easily killed by many things in a human-scale world. Natural predators (cats, birds, snakes, etc.) will be an issue as well. Employers will be hesitant to invest too much into a gecko worker because they know there are a hundred mundane, everyday things that could kill or maim them.

This all means that the entirety of their education and vocational training will be compressed into perhaps 3 years tops. You can expect a gecko worker to have basic literacy, some fundamental skills related to their job, and not a whole lot else. An 8-9 year old human would outclass them educationally in every way. Thus, they will be limited to unskilled labor or things that exploit their biological differences. Tasks that require higher-order thinking skills or extensive education would be completely impractical.

A few examples of things a gecko could plausibly do:

• They would be excellent night watchmen or lookouts for a military fort. Their night vision is orders of magnitude better than a human's, and many are naturally nocturnal. Some can even focus on two different objects at different depths at the same time.
• Simple toolwork tasks in an assembly-line setting. Their ability to fit into small spaces could enable the manufacturing of designs that would not be possible for humans in a pre-industrial setting. In particular, they could carve features into the inside of wooden objects. This can enable things to be built out of a single solid piece of wood instead of multiple smaller pieces joined together (akin to assembling the ship inside the bottle).
• Inspection of hard-to-access locations. A gecko can climb up a wall and onto a roof in a fraction of the time it would take a human. They don't need a ladder, are extremely unlikely to fall, and even if they do fall their injuries would be significantly less than a human's. They can easily inspect places like the inside of a wall cavity which would be a destructive process for a human. They can scour a ship's hull to locate tiny leaks that would be hard for a human to spot without magnification.
• They could be part of an authentication or anti-counterfeiting system used by banks, courts, lawyers, etc. Geckos can see into the ultraviolet. Currency or documents can be sealed with ink that can only be seen and interpreted by a specially-trained gecko with access to a proof set. Humans would have a hard time forging a seal that they can't see. Geckos would have a hard time forging the seal because they would be made too large for a gecko to use.
• Basic pest control, a.k.a. "employer sponsored free lunch". You probably wouldn't even have to pay them (I have such an arrangement with the geckos on my porch and they happily do it for free).
• Small-scale marking and measuring tasks. Their claws are sharp enough that they could leave a faint scratch mark on soft wood or some types of fabric, or they can use their tail as an indicator to a human with a pencil. They can measure by walking off paces like a human would. Paces can be rather accurate with a bit of practice (e.g., armies, marching bands, and Alexander the Great's bematists).

So yes, there are jobs that a gecko could do. There's probably not enough of those sorts of jobs to employ an entire gecko civilization, though. Your human population would have to subsidize the rest of the gecko population, which shouldn't be a burden given their low maintenance costs compared to a human.