Hi @mccoole,
The thing is that when you use gitlab as a SSO you won't have a password assigned as the only way to authenticate the user is by using gitlab.
So as a workaround maybe you will want to enable the OAuth provider built-in Mattermost and have the user authenticate using OAuth which in turn will ask you to authenticate against gitlab, but then you'll be provided with an access_token that you can use to make further API calls.
let me know if this helps.
Cheers