Gitea is finally in the process of moving off of GitHub to develop Gitea using Gitea: https://github.com/go-gitea/gitea/issues/1029#issuecomment-457425862
We recently set up a Gitea for Kosmos, which we also intend to open to other FOSS projects in the future. Already enjoying it a lot, even though it's a bit rough around the edges still. Core functionality is all there, and also some features GitHub is missing as of now (e.g. native WIP status for PRs, issue dependencies, and others).