Hi @Juraj, thanks for the question.
Unless user creation is a process in which a GitLab auth-ed user gets a Mattermost account, in which case user creation should be enabled, but limited to GitLab?
That's correct. In the System Console, you'll want to reenable account creation under General > Users and Teams, but then disable account creation with email under Authentication > Email. After that, users will be able to create their Mattermost account only if they have an existing Gitlab account to link it to.
You'll probably also want to disable sign-in with email and sign-in with username on Authentication > Email as well to hide the username and password boxes on the login page.
If you're running Mattermost as part of Gitlab Omnibus, these settings are available in gitlab.rb as
mattermost['enable_account_creation'] = true
mattermost['email_enable_sign_up_with_email'] = false
mattermost['email_enable_sign_in_with_email'] = false
mattermost['email_enable_sign_in_with_username'] = false