Swift-function for finding missing Integers in an array of sequential Integers

In the spirit of the Swift API Design Guidelines, in particular

Prefer method and function names that make use sites form grammatical English phrases.

I would call the method

func missingNumbers(in numbers: [Int]) -> [Int]

so that it is used as

print(missingNumbers(in: arrTest))

Comparing a boolean value with true or false is unnecessary, i.e.

if numbers.contains(i) == false { ... }

is (in my opinion) better written as

if !numbers.contains(i) { ... }

Instead of an explicit loop and a temporary array we can apply filter to a range:

func missingNumbers(in numbers: [Int]) -> [Int] {
    return (1...100).filter { !numbers.contains($0) }
}

which is concise but still easy to read.

(The return keyword can be omitted if the function body consists of a single expression, that is a matter of personal taste.)

If many containment test are done against the same collection of values then it can be advantageous to convert the array to a Swift Set first because Set.contains() is a \$ O(1) \$ operation. It probably makes no performance difference for 100 values, but this is what it would look like:

func missingNumbers(in numbers: [Int]) -> [Int] {
    let set = Set(numbers)
    return (1...100).filter { !set.contains($0) }
}