I just saw two tutorials one with Google Play Services to login and then use achievements, ladder, etc. So it has compile 'com.google.android.gms:play-services-plus:8.4.0' to login.
And in Firebase tutorial we use 'compile 'com.google.firebase:firebase-auth:9.0.0'
Isn't that similar services to connect and we can't use both in one? It would be big gap in UI/UX if user would connect two times to similar service.
Question: Could I use one auth for both?
You should understand that firebase was once an independent organisation and but later on acquired by google. The earlier document were found at Firebase Previous Docs but now its part of google and google and accessed here Firebase Google Current Docs. So even before acquiring firebase google services were there stuffs like GCM google cloud messaging which is now firebase cloud messaging. So it turns out that after for authentication using firebase can be from various accounts like google,facebook,twitter,github etc but in that example of yours you have an option of logging in as google account only because is a google plus. So consider to stay updated, firebase is simple and current while the other will be completely deprecated later.
