We are working on integrating GitLab (enterprise edition) in our tooling, but one thing that is still on our wishlist is to create a merge request in GitLab via a command li
As of GitLab 11.10, if you're using git 2.10 or newer, you can automatically create a merge request from the command line like this:
git push -o merge_request.create
More information can be found in the docs.