The git+nginx would suffice but it does not offer GUI. I need one to see the changes proposed (aka PRs).
Gitea is nice, but a bit overkill for my needs. I don't need CI, files hosting, issues, team members, releases, wiki, forking/watching/staring, etc.
I'm just looking for "website" (read: interface) that list files over HTTPS, with the ability to show nice looking diffs. Some sort of ssh keys(?) to prevent unauthorized access etc.
https://github.com/sitaramc/gitolite
I've not used it personally though, so no idea how well it works in practise.
I just found about "git instaweb" (developed by Git authors, but it's extra OS package) - it works locally, probably what I was looking for (minus the 100% width page layout).
BTW There's Gitblit is someone's fancy hosting Java app.
I have not used it before, but it seems like it follows your requirements (Except auth using SSH keys maybe, but that could be a task for e.g. Nginx)
Depending on your needs, this can be as simple as sticking repos on any server you have and cloning/pulling/pushing over ssh. If you want something more sophisticated, though, there's a handful of nice applications (gitea is being suggested further up-thread).