How did you create your fork?
1) by pressing the fork button in the web interface of Gitlab?
2) by making a local clone, and then importing this to a new Gitlab.
3) by downloading sources, and creating a new git from it (not a fork)
In case of 1 it should work. 2, I have never tested.
In case of 1, when you go to "new merge" request, it should have one your branches pre-selected on the left, and one of the lazarus repo on the right.
In case of 2 see the post by Thausand.
Or never mind that... If it doesn't work, you can add a normal issue, with a link to your branch (and a note that gitlab didn't let you do a merge request). That would be a classic "pull request" => a request to pull your branch, and integrate it.
That is ok in case of 2, but not 3.
In case of 2 commits that came from the original repo, have the same sha1.
So when any dev pulls your branch, git will see the matching sha1, and it becomes a normal branch in their local repo.
I do that with most merge requests too, so I can test them before merging.
It's a bit more work for the developer, without a merge request, because your commits need to be changed, so the committer will be a developer (or they can't be pushed, due to security settings). Mind, only the committer. The author is kept as you.
So it depends who of us would be to look at them, and how git savvy that person is.