Initialization of variable

private GoogleApiClient mGoogleApiClient;

You must have to Write this Code in onCreate() method of all that when u put signout button.

mGoogleApiClient = new GoogleApiClient.Builder(this)
            .enableAutoManage(this /* FragmentActivity */, this /* OnConnectionFailedListener */)
            .addApi(Auth.GOOGLE_SIGN_IN_API)
            .build();

Put below code on signout button.

Auth.GoogleSignInApi.signOut(mGoogleApiClient).setResultCallback(
                    new ResultCallback<Status>() {
                        @Override
                        public void onResult(Status status) {
                            FirebaseAuth.getInstance().signOut();
                            Intent i1 = new Intent(MainActivity.this, GoogleSignInActivity.class);
                            startActivity(i1);
                            Toast.makeText(MainActivity.this, "Logout Successfully!", Toast.LENGTH_SHORT).show();
                        }
                    });