Login.kt

class Login : AppCompatActivity() {

    //Google Login Request Code
    private val RC_SIGN_IN = 7
    //Google Sign In Client
    private lateinit var mGoogleSignInClient: GoogleSignInClient
    //Firebase Auth
    private lateinit var mAuth: FirebaseAuth



    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_login)

        mAuth = FirebaseAuth.getInstance()

        // Configure Google Sign In
        val gso = GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                .requestIdToken(getString(R.string.default_web_client_id))
                .requestEmail()
                .build()


        mGoogleSignInClient = GoogleSignIn.getClient(this,gso)

        sign_in_button.setOnClickListener({
            signIn()
        })

    }

    private fun signIn() {
        val signInIntent = mGoogleSignInClient.getSignInIntent()
        startActivityForResult(signInIntent, RC_SIGN_IN)
    }

    public override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent) {
        super.onActivityResult(requestCode, resultCode, data)

        // Result returned from launching the Intent from GoogleSignInApi.getSignInIntent(...);
        if (requestCode == RC_SIGN_IN) {
            val task = GoogleSignIn.getSignedInAccountFromIntent(data)
            try {
                // Google Sign In was successful, authenticate with Firebase
                val account = task.getResult(ApiException::class.java)
                firebaseAuthWithGoogle(account)
            } catch (e: ApiException) {
                // Google Sign In failed, update UI appropriately
                Log.w("Login", "Google sign in failed", e)
                // ...
            }

        }
    }

    public override fun onStart() {
        super.onStart()
        // Check if user is signed in (non-null) and update UI accordingly.
        val currentUser = mAuth.currentUser
        updateUI(currentUser)
    }

    private fun firebaseAuthWithGoogle(acct: GoogleSignInAccount) {
        Log.d("Login", "firebaseAuthWithGoogle:" + acct.id!!)

        val credential = GoogleAuthProvider.getCredential(acct.idToken, null)
        mAuth.signInWithCredential(credential)
                .addOnCompleteListener(this) { task ->
                    if (task.isSuccessful) {
                        // Sign in success, update UI with the signed-in user's information
                        Log.d("Login", "signInWithCredential:success")
                        val user = mAuth.currentUser
                        updateUI(user)
                    } else {
                        // If sign in fails, display a message to the user.
                        Log.w("Login", "signInWithCredential:failure", task.exception)
                        Toast.makeText(this,"Auth Failed",Toast.LENGTH_LONG).show()
                        updateUI(null)
                    }

                    // ...
                }
    }

    fun updateUI(user: FirebaseUser?){
        if(user != null){
            //Do your Stuff
            Toast.makeText(this,"Hello ${user.displayName}",Toast.LENGTH_LONG).show()
        }
    }

}