Hi @kingfisher77, you would need to have each user manually switch their account from a local email/password account over to a GitLab OAuth account. There's a couple reasons for this:
- We just haven't implemented anything with import that will deal with SSO
- There really isn't anyway to do the import and have all accounts already setup with their SSO, since we need each user to sign-in with their GitLab account to verify it's actually the owner of that account
That said, we have been discussing admin features that would allow the system admin to switch every user's sign-in method to one type of SSO (GitLab for example). Then any use not using that SSO type, would then be logged out and asked to claim their account with SSO. A feature like this would certainly help out your situation as well as give the admin some options for controlling access to Mattermost.