Is there really nothing like BitTorrent for git, or have we just not heard about it because of GitHub's network effects? It feels like this problem was solved long ago for binaries.
gittorrents were talked about and built at least 15 if not 20 years ago.
the issue isn't mirroring of data, this is a solved problem. everything else that a forge does is a problem - issue tracking, PRs, reviews, CI/CD, authn, authz, secrets, audit trails, ...
Radicle may be what you're after
Git is already distributed by itself. The management-part is what's missing (mergerequests, permissions, issues..), and it's disputable whether this is really necessary, or just a nice to have.
There is! https://radicle.dev :)