What do people mean by "only google account supported", I've been building a web app with app engine and I just ran my own auth using passlib and it's worked fine. I also don't see how they can prevent you from adding oauth from different services onto your app.

The problem I ran into was that with python you could only use certain native libraries provided by google. To use third part unsupported libraries in python you had to do a hack where you submit them with the app but I managed to get it working with a bit of fiddling.