HI there! I hosted GitLab on digital ocean, and it takes more than 4 gb of memory, but requirements say 4 gb must be enough. Why so much? 
Official Gitlab documentation mentions 4vcpu, 8GB as minimal specs. Running any less than this is possible, but means disabling a lot of stuff to run it in a memory constrained environment. This is explained in the Gitlab docs for running on a Raspberry PI. I have done it with 4GB, it is possible. Preferred is 8GB though.