GitLab with Custom OmniAuth data

I saw that I can use GitLab with custom OmniAuth providers. I have a custom OAuth2 system set up, so omniauth-oauth2 looks great. My OAuth2 API will return the user’s name, email, username, base64 encoded picture, groups, and whether or not they are an admin (as well as an admin group). I know already that I can probably use name, email, and username, and I don’t care too much about the picture. However, could I get GitLab to automatically identify the groups and admins through this OmniAuth provider? Basically, what is the big list of user attributes that GitLab will recognize through OmniAuth?

Thanks,
DaAwesomeP

Is this not possible?

Opened issue about lack of detailed documentation for custom OmniAuth providers: https://gitlab.com/gitlab-org/gitlab-ce/issues/2927