This is another piece of code I've extrapolated from a Ruby on Rails project I'm currently working on. The code implmenets social login with a RoR API-based application, targeted at API clients.
The setup does not involve any browser-redirects or sessions as you would have to use working with Omniauth.
Instead, what it does is takes an access_token generated on client-side SDKs, retireves user info from the access token
and creates a new user and Identity in the database.
This setup works with native applications as described in the Google iOS Sign In Docs (see Authenticating with a backend server)