GIDSignIn.swift

guard let clientID = FirebaseApp.app()?.options.clientID else { return }

// Create Google Sign In configuration object.
let config = GIDConfiguration(clientID: clientID)

// Start the sign in flow!
GIDSignIn.sharedInstance.signIn(with: config, presenting: self) { [unowned self] user, error in

  if let error = error {
    print(error)
    return
  }

  guard
    let authentication = user?.authentication,
    let idToken = authentication.idToken
  else {
    return
  }

  let credential = GoogleAuthProvider.credential(withIDToken: idToken, accessToken: authentication.accessToken)

  Auth.auth().signIn(with: credential) { authResult, error in
    if let error = error {
      print(error)
      return
    }
    // User is signed in
    passToMainView()
  }
}