Avoid external users to be able to use Gitlab Application Authentication to login into Mattermost

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I don't know if this is the right place for this problem, neither if it is a feature proposal or I missed something in the gitlab configuration.

The problem is the following, we use Gitlab CE to provide the user accounts and the authentication for our Mattermost instances. Now we came up with the problem that we have some users, marked as external ones in Gitlab, that should not be able to access mattermost. So the question is can we restrict Gitlab's authentication provider such that external users do not get a valid authentication token and thus cannot access Mattermost?