Android facebook sdk with manual gatekeeper or inventor

I have a more structured question about constructing a connection between application (android) and server (rails-api gem) using json. It has to use facebook as authentication and then handle the connection flow through the token.

The planned solution looks like this:

• I am planning to use facebook sdk (client side) to get facebook access token.
  a) Should I use the client side? I am considering this because the request will not be fulfilled by my server. But is the access token trustworthy?

• After that, I send the access token to my server and validate it using the facebook graph

• If the token is correct, I create a new user with information from facebook graph

• Next, I use a gatekeeper to create an oauth provider. The client receives an oauth2 access token which it uses to communicate with the server ...
  a) Should I use a different gem for authentication? Or should I use the facebook access token as my normal authentication token?
  b) I'm not sure if I should use a gatekeeper or invent. Are there enough pre-markers? I read somewhere that u should use a gatekeeper for apis and develop regular websites.
  c) I am not getting hidden grant stuff from oauth2. Should I count it here?

Thanks for the help:)