@Brendanjones @erlend @fediversenews
A good site for background is https://forgefed.org where a specification for an AP extension is crafted. Implementers of the spec can exchange merge requests between code forge softwares. Another forge that has received @NGIZero funding to add federation support is Gitea. It's downstream fork Forgejo is also working on that.
On the Gitlab epic the question was asked whether @forgefed support is planned.