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, ...
BitTorrent also enabled search engines to be built easily, which created discoverability. Unfortunately it's a much harder problem for git repos, especially when competing with GitHub search.