Sorry if this doesn't qualify. I used the first version years ago when I had files on Google.
https://github.com/vitalif/grive2
I don't really know what a custom oauth setup means, and, apparently, it doesn't yet support Google Docs, which may be a must for you.
@bashfulrobot the gnome online accounts thing works on NixOS just fine, even with multiple accounts
@uep yeah that’s what I’ve been using as a back up right now. It just doesn’t work very well opening files from there for editing. (Laggy). I edit a number of markdown files in the drive.