How to set up a code review using Gitlab?

Note: since GitLab 6.4, side-by-side diff view is available: see "pull request 5308".

(July 2013)There is no possibility yet to comment on each line though, only at the file level.
Daniel Sokolowski mentions in the comments that Per line comments are now supported (09/2014):

Your team members can comment on the merge request in general or on specific lines with line comments.

That still can help for code review activity.

6 years later, for GitLab 13.1 (June 2020):

Merge Request Reviews moved to Core

Originally introduced in GitLab 11.4 as a GitLab Premium feature, Merge Request Reviews allow merge request reviewers to:

• submit multiple comments at once,
• cutting down on notification noise for the merge request author, and
• allowing for a more cohesive and streamlined review process.

Since its introduction, we’ve re-evaluated its place in our buyer-based pricing model and as part of 13.1 we’re excited to announce this feature has now moved to GitLab Core.

See Documentation and Issue

With GitLab 13.9 (February 2021), you also have:

Request a follow-up review from a Reviewer

Merge request authors receive feedback from reviewers. Often, this feedback needs to be re-reviewed to make sure all issues have been appropriately addressed. As the author of the contribution, you need to communicate the need for a follow-up to your reviewer.

For reviewers who have already reviewed a merge request, you can now request a re-review. This request triggers a To-Do item and email notification to alert the user that you need a follow-up review of your contribution.

See Documentation and Issue.

With GitLab 13.11 (April 2021)

Add standalone comments to merge request reviews

When you’re reviewing changes, you may want to summarize your feedback, or comment on something unrelated to the specific changes. Reviews in GitLab only allowed commenting on the changes or replying to existing comments, which meant that other kinds of comments had to be made after submitting the review.

As part of your review process, you can now submit a general comment along with your replies or change-specific comments. General comments make it simple to provide feedback to the author and use quick actions to update items across the merge request.

Thanks to Lee Tickett for the contribution!

See Documentation and Issue.

I've been doing code reviews in Gitlab for over two months with almost no friction. I've setup rss2email to send email notifications everytime a developer pushes new commits. Then I use Gitlab's comment feature for commits to make some comments about the pushed code.

Unfortunately, Gitlab does not allow comments on files itself, only in commits (just like Github, I guess). Whenever I find myself in a situation that I need to comment something that I've missed in a previous commit, I use the blame tool to find the commit that introduced/changed the code section to be commented.

It is far from perfect, but it is working good so far.

You can see submitted code in Merge Request for other repository or in current repository.
example http://demo.gitlab.com/diaspora/diaspora/commits/master

Then you can add comments to committed file changes (button Reply) or to the whole commit

example http://demo.gitlab.com/diaspora/diaspora/commit/42f47626890218a180870bc3f44ec57625b0779c

The resulting communication is code review. However, I personally recommend to do code review on one PC with face-to-face communication whenever possible, and use tools for recording results or when more formality is needed.

For a file revue that has a lot of commits, e.g. http://demo.gitlab.com/diaspora/diaspora/blame/master/README.md look at it using blame to understand who did what. However in this view there is no option to communicate and add comments. I would recommend just add changes as comments in this case.

In case this helps someone in the future:

One of the limitations of GitLab code review is that it only allows commenting on files that are part of a merge request. This works great for incremental changes to projects, but for reviewing an entire repo it's not helpful.

I just needed to do a large code review of a new project and wanted to see all of the files in the review (not just to specific commit), and came up with a workaround: I went to the root of the commits and created a branch called 'initial'. I then created a merge request for 'initial' -> 'main'. This gives me an MR with the latest state of every source file in the repo, and I can now do a code review, add comments, etc... on the entire source tree.

Yes. Merge requests are how peer reviews are accomplished.

There should be a 'diff' tab that will show the changes of all the commits (mentioned here: http://youtu.be/DyAX8ws5OIc?t=3m2s).

The video also explains nicely how it can be used to peer review.

The normal use-case of code reviews is to review code on a branch before merging into master or similar. I have a situation where I've developed a project and want all the code to be reviewed by everyone on the team.

What I did was:

Checkout the first commit, make a change to it, commit and push

git co -b FIRST_COMMIT eb67f06c2b3222c0219214b176c41922bc454881
vi README.md
git add README.md
git ci -m "First commit modified so can get full diff against it"
git push --set-upstream origin FIRST-COMMIT

Checkout the last commit, make a change to it, commit and push

git co -b master
vi README.md
git add README.md
git ci -m "Last commit modified so can get full diff against it"
git push --set-upstream origin LAST-COMMIT

On GitLab / GitHub, create a pull request

• It is one merging from LAST_COMMIT to FIRST_COMMIT

Works for me!