The git:// protocol requires its own daemon (which is bundled with Git) running on port 9418 https://git-scm.com/book/en/v2/Git-on-the-Server-The-Protoco... . The standard Git daemon is unsuitable for GitLab I think because of the way it manages access control.

https://git-scm.com/book/en/v2/Git-on-the-Server-Git-Daemon

For this to work in GitLab we would need to find or write an alternative implementation of the protocol.

Sounds like a lot of work to me when we already have decent HTTP clone access.