sign-in using Googl€
We can sign-in Mattermost using Gitlab. That's great!
Some team members may use a Google (account) (that happen) without having a Gitlab account.
For them, joining a team via
FramaTeam > Gitlab > Google, fails on
Signing in using your GitLab.com account without a pre-existing GitLab account is not allowed
Offering a direct Googl€ [insert any adjective] would offer a (real) incentive for teams to use a Slack-alternative without mandating a change of their OAuth provider.