I know that Generic OAuth isn't supported. However, as far as I can tell, GitLab uses a completely generic OAuth2 implementation, and I saw some references to "unsupported use" of GitLab as being possible.
I have signed up for a GitLab account to see if it works, but I expect no issues there.
I am very happy to make some changes to the GitLab module, but am not entirely sure how to debug the calls MM is making.