I think this is a purely business move by Amazon. They see money in game industry, they want to tap into that. Unity is making money, UE4 is making money, hey, they have AWS which is already used by some game developers. Why not have an engine that's deeply integrated to the AWS services?
You can now do shits like notify your players accomplishments, "useful" game analytics, twitch integration, etc etc.
I can feel their marketing strategy for this engine will be very corporate and enterprisey. Their "Solutions Architect" will stand there and sell this engine like selling a new Big Data platform.
Sure, but GitHub/GitLab is so much more than bare git -- Issue tracking is the big one (aside from, again, having a community of users). You can spin up some other solutions for that too, or maybe use something like Trello or Slack, but the nice thing about GitHub/GitLab is that its all in one box so-to-speak, and the cross-integration is nice -- tying issues to commits, or shouting out to other contributors twitter-style.
That's basically what you are paying for, the UI frontend with the issue tracking and the project collaboration. Not saying it's a bad thing to have or pay, just letting people know that you can still have your own private git repo, if you don't need the UI frontend and the issue tracking. If you set your server repo right, you can have multiple users use your private repo.
Do you guys realize that you don't need any special software to have a remote git repository? Spin up a server somewhere (at your home, whatever), run sshd, open up port 22, create your user, install git. Pick a location on your server (e.g /opt/myrepo) accessible by that user, run git-init with bare options. Done.