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Gogs uses a mix of GoGits, a partially Go implementation of git, and the traditional git binary via os/exec. A quick scan shows that it relies on the binary for perhaps the majority of tasks.

Another project I'm aware of that aims to re-implement git in go is https://github.com/kourge/ggit, which looks promising but has been slowing down, activity-wise.

There's also dulwich, which is in Python: https://github.com/jelmer/dulwich

bup, the deduplicating backup software, uses the git pack format for storing its data (because the tree structure of git matches really well the tree structure of how bup operates). They have written their own implementation of pack writers and readers [0]; the writers are pure-git, the readers call the git binary.

[0] https://github.com/bup/bup/blob/master/lib/bup/git.py

There is js-git: https://github.com/creationix/js-git