When talking about users authentication in Rails land there is one name that generally stands above all the other available gems. This name is Devise.
I would not be so wrong to call it the de facto standard of Rails authentication considering the time has been around and the vast documentation it has under its belt.
Regarding for example the OAuth2 functionality there is a well documented page inside the wiki that describes how to implement it for Facebook.
Unfortunately what presented inside the documentation doesn’t always blend well with the other functionalities of an application.