I wonder if it's practical to introduce a federation backend into GitLab? Secondly, I guess it's also worth discussing which bits would actually federate. User profiles? Git commits and comments? Branches? How would things like forking and merging branches and repos work across servers?

I think Red actually supports some kind of git frontend, or at least I think I've seen people using it in the past. In theory it wouldn't be too difficult to create a plugin to support it.