Creating pull requests in gitlab

GitLab.com co-founder here. Forking should work fine in recent versions of GitLab (6.x). You can fork a repo belonging to someone else and then create a merge request (the properly named version of the GitHub pull request).

As mentioned in "Development" and in this thread

There is no forking in GitLab (at least until GitLab 5.2, May 2013 as mentioned by, and thanks to Angustus)
because it's not meant to have that kind of functionality like GitHub.
If you're using GitLab, presumably you're going to either own the repo or someone on your team, in which you would have access and can create a branch to work on a feature / whatever.

If you create a branch, then you can submit a merge request where it can be reviewed by other people in the project (or whatever your workflow dictates), and accept the merge.

Accepting merge request has been implementing in Issue 618:

GitLab will have forking as of version 5.2. Cross repo pull requests will soon follow.

Fork pull request: https://github.com/gitlabhq/gitlabhq/pull/3597