If you're running Mattermost 1.3+ you can do the following:
Ok, as an admin you don't have direct access through the UI to change the user's email address (this is a feature we'd like to add as we expand the functionality of the System Console).
You can fix this by permanently deleting the user's Mattermost account (don't worry it won't touch their GitLab account). The command to do this is:
platform -permanent_delete_user -team_name="name" -email="user@example.com"
They should then be able to go and create a new account using GitLab SSO.
IMPORTANT NOTES
- This will totally wipe out any posts that the user has made before.
- Strongly suggest backing up your DB before attempting this.
There is also the possibility of manually updating the email address to something random so you don't have to delete the user but we super strongly suggest no one messes directly with the DB.