I am looking into GitLab and Bitbucket, but I don't plan on supporting generic Git, unless I can find a way to slap an API in front of it. I'll add that to the backlog.

Maybe you could add support for Gitea? It’s a pretty handy way to slap an API in front of git and it’s much simpler to self host than GitLab.

Aspects of the API look fairly similar to GitHub actually. I'll look into it.

How much of your API is dependant on GIT like semantics and how much could work with "plain files" storage?

It's neat to be able to transplant a CMS on top of other services that don't necessarily deploy by CI pipes, I made a DAV plugin(basic auth) for Netlify (back before the forks) so as long as a server supported listing(only DAV dependency really), GET, PUT and DELETE you could just plunk down the CMS files somewhere and point to the DAV share.

I believe Netlify CMS has an actual backend interfacing with the various Git systems they supported. Pages CMS is different in that it's a pure front-end app. I have a couple tiny serverless functions to help authenticate you with GitHub, but then after that there is no backend involved. That means I need some API for the front-end to interact with, at least for now.