The GitLab 'internal' API is exposed unauthenticated on GitLab. This allows the username for each SSH Key ID number to be retrieved. Users who do not have an SSH Key cannot be enumerated in this fashion. LDAP users, e.g. Active Directory users will also be returned. This issue was fixed in GitLab v7.5.0 and is present from GitLab v5.0.0.
To display the available options, load the module within the Metasploit console and run the commands 'show options' or 'show advanced':
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– Jim O’Gorman | President, Offensive Security