Hello,
Thank you for using Google Groups!
Based on my understanding of what you have described, you can implement OAuth within Google App engine. You may find further details of this
here.
However, I was able to find
StackOverflow threads that revolve around this issue with Webapp2. In regards to specific social logins, I was able to find the following thread
here that discusses how to integrate Facebook login within App Engine. Though, I believe it would be worthwhile for you to check out the discussion
here as the entire thread discusses "Webapp2 Authentication and Login". Additionally, there is a third party boilerplate provided by the community on that thread for what you are attempting to do. You may feel free to explore the links shared to determine if the ideas discussed in them would be applicable in your use case.
I hope this helps!