That's a really good question!

Has anyone here used the self-hosted version of GitHub Enterprise? Did they really reimplement git?

Edit: Apparently GitHub uses https://libgit2.org/ which is "GPLv2 with a Linking Exception" (equivalent to LGPL)

My understanding of GHE is that it uses libgit2 internally, which in turn has a linking exception in its license (just like GCC and glibc). It’s unlikely that they’re violating GPL in that particular way.

GitHub Enterprise is proprietary software. To the best of my (external observer's) knowledge, it has no GPL code in it. If it does, then that's probably a licensing violation, but IANAL.