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 now, GitLab sadly does not support this, however I recently saw it on their issue tracker. It appears one can expect a 'native tool' in the upcoming months.
GitLab tweeted out about numa08/git-gitlab some time ago, so I guess this would be worth a try.