Introduced in GitLab 8.7. It is OFF by default because it still causes too many false alarms.
Git has a built-in mechanism, git fsck, to verify the integrity of all data committed to a repository. GitLab administrators can trigger such a check for a project via the project page under the admin panel. The checks run asynchronously so it may take a few minutes before the check result is visible on the project admin page. If the checks failed you can see their output on the admin log page under 'repocheck.log'.
When enabled, GitLab periodically runs a repository check on all project repositories and wiki repositories in order to detect data corruption problems. A project will be checked no more than once per month. If any projects fail their repository checks all GitLab administrators will receive an email notification of the situation. This notification is sent out once a week on Sunday, by default.
You can disable the periodic checks on the 'Settings' page of the admin panel.
If the repository check fails for some repository you should look up the error
in repocheck.log (in the admin panel or on disk; see
/var/log/gitlab/gitlab-rails for Omnibus installations or
/home/git/gitlab/log for installations from source). Once you have
resolved the issue use the admin panel to trigger a new repository check on
the project. This will clear the 'check failed' state.
If for some reason the periodic repository check caused a lot of false alarms you can choose to clear ALL repository check states from the 'Settings' page of the admin panel.