The Gitaly server has a Ruby component, but also a Go component. The Ruby server uses Rugged[1] and Gollum-lib[2] which both use libgit2.
The Go component doesn't have libgit2 binding yet, although we're looking into adding that later. That or maybe go-git[3]. But for now Gitaly is mainly focussed at migrating all git calls from the Rails monolith. Not introducing a new component now reduces the risks this project has.