Get google token

This is my solution:

First setup playgames configuration like this

 PlayGamesClientConfiguration config = new PlayGamesClientConfiguration.Builder()
             .RequestIdToken()
             .Build();

 PlayGamesPlatform.InitializeInstance(config);

 PlayGamesPlatform.Activate();

Then login using

 Social.localUser.Authenticate

Now you can get token

 googleIdToken = ((PlayGamesLocalUser)Social.localUser).mPlatform.GetIdToken();

and use it to login to firebase

 Credential credential =
             GoogleAuthProvider.GetCredential(googleIdToken, null);
             auth.SignInWithCredentialAsync(credential)

I hope this helps