Setup Mirrored Repo in Github

I recently learning about a gnu app and wanna see the source code so searched it in google and I found this.

This repo is not hosted in GitHub but hosted in git:// but still visible in GitHub because either GitHub or the organisation mirrored it on GitHub.
I done little research in docs but didn’t got any success.

Can anyone tell me if this feature is available public or just to some organisation of GNU.
And how to setup this.

I guess this repo doesn’t consists any remote origin link and just contain remote upstream link in .git
But i doubt.