How to Signout a user in Flutter with Firebase authentication

You need to have an Instances of FirebaseAuth

  final FirebaseAuth _firebaseAuth = FirebaseAuth.instance;

And Then

  _signOut() async {
    await _firebaseAuth.signOut();
   }

Firebase auth's signOut method is asynchronous. You should make your _signOut method async.

Future<void> _signOut() async {
  await FirebaseAuth.instance.signOut();
}

so that the call to runApp occurs after the user is signed out.

You should also call _googleSignIn.signOut() when logging out if you want signIn to present the user with an authentication dialog instead of silently and automatically re-using the current Google user.