Add below to your build.gradle out of android tag:

// Apply plug-in to app.
apply plugin: 'com.google.gms.google-services'

Add below helper class to your util package:

/**
 * Created by Andy
 */
public class GoogleSignInHelper implements GoogleApiClient.OnConnectionFailedListener,
        GoogleApiClient.ConnectionCallbacks {
    private static final String TAG = GoogleSignInHelper.class.getSimpleName();

    private static GoogleSignInHelper googleSignInHelper;
    private AppCompatActivity mActivity;
    private GoogleApiClient mGoogleApiClient;
    public static final int RC_SIGN_IN = 9001;
    private boolean isLoggingOut = false;

    public static GoogleSignInHelper newInstance(AppCompatActivity mActivity) {
        if (googleSignInHelper == null) {
            googleSignInHelper = new GoogleSignInHelper(mActivity, fireBaseAuthHelper);
        }
        return googleSignInHelper;
    }

    public GoogleSignInHelper(AppCompatActivity mActivity) {
        this.mActivity = mActivity;
        initGoogleSignIn();
    }


    private void initGoogleSignIn() {
        // [START config_sign_in]
        // Configure Google Sign In
        GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                .requestIdToken(mActivity.getString(R.string.default_web_client_id))
                .requestEmail()
                .build();
        // [END config_sign_in]

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

    }
    
    @Override
    public void onConnectionFailed(@NonNull ConnectionResult connectionResult) {
        // An unresolvable error has occurred and Google APIs (including Sign-In) will not
        // be available.
        Log.d(TAG, "onConnectionFailed:" + connectionResult);
        Toast.makeText(mActivity, "Google Play Services error.", Toast.LENGTH_SHORT).show();
    }

    public void getGoogleAccountDetails(GoogleSignInResult result) {
        // Google Sign In was successful, authenticate with FireBase
        GoogleSignInAccount account = result.getSignInAccount();
        // You are now logged into Google
    }
    public void signOut() {

        if (mGoogleApiClient.isConnected()) {

            // Google sign out
            Auth.GoogleSignInApi.signOut(mGoogleApiClient).setResultCallback(
                    new ResultCallback<Status>() {
                        @Override
                        public void onResult(@NonNull Status status) {
                            isLoggingOut = false;
                        }
                    });
        } else {
            isLoggingOut = true;
        }
    }

    public GoogleApiClient getGoogleClient() {
        return mGoogleApiClient;
    }

    @Override
    public void onConnected(@Nullable Bundle bundle) {
        Log.w(TAG, "onConnected");
        if (isLoggingOut) {
            signOut();
        }
    }

    @Override
    public void onConnectionSuspended(int i) {
        Log.w(TAG, "onConnectionSuspended");
    }
}

Add below code to your OnActivityResult in Activity file:

 // [START onactivityresult]
    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        // Result returned from launching the Intent from GoogleSignInApi.getSignInIntent(...);
        if (requestCode == GoogleSignInHelper.RC_SIGN_IN) {
            GoogleSignInResult result = Auth.GoogleSignInApi.getSignInResultFromIntent(data);
            if (result.isSuccess()) {
                googleSignInHelper.getGoogleAccountDetails(result);
            } else {
                // Google Sign In failed, update UI appropriately
                // [START_EXCLUDE]
                Log.d(TAG, "signInWith Google failed");
                // [END_EXCLUDE]
            }
        }
    }
    // [END onactivityresult]

 // [START signin]
    public void signIn() {
        Intent signInIntent = Auth.GoogleSignInApi.getSignInIntent(googleSignInHelper.getGoogleClient());
        startActivityForResult(signInIntent, GoogleSignInHelper.RC_SIGN_IN);
    }

    // [END signin]