Use Github or Git Gateway backend on a site hosted on AWS S3

Is it possible to use Github or Git Gateway as the backend on a NetlifyCMS site hosted on AWS S3?

Sure! Git gateway is open source, so you can definitely download and host your own copy, though we don’t provide any tech support for that:

The easier pattern would probably be to have us host the CMS for you (we’d host a copy of your site, that is attached to your git repo), and then you can still use S3 or whatever hosting service you want to use for your production traffic, pulled from the repo that the CMS feeds into.