n00b: authentication via Google

AlexD wrote in post #1074276:

What is the simple way in Rails to do user authentication via Google?

OmniAuth:
https://github.com/intridea/omniauth

Authentication strategies:
https://github.com/intridea/omniauth/wiki/List-of-Strategies

Google specific strategies:
https://github.com/Yesware/omniauth-google
https://github.com/zquestz/omniauth-google-oauth2