Ionic Google OAuth Authentication, Firebase 3 and (ngCordovaOauth plugin or cordova-plugin-googleplus)

app.js

angular.module('App', ['ionic', 'ngCordova', 'ngAnimate', 'ngCordovaOauth', 'firebase'])

.run(['$ionicPlatform', 
      '$rootScope',
      '$firebaseAuth',
      function($ionicPlatform, $rootScope, $firebaseAuth) {

  $ionicPlatform.ready(function() {
    if(window.cordova && window.cordova.plugins.Keyboard) {
      // Hide the accessory bar by default (remove this to show the accessory bar above the keyboard
      // for form inputs)
      cordova.plugins.Keyboard.hideKeyboardAccessoryBar(true);

      // Don't remove this line unless you know what you are doing. It stops the viewport
      // from snapping when text inputs are focused. Ionic handles this internally for
      // a much nicer keyboard experience.
      cordova.plugins.Keyboard.disableScroll(true);
    }
    if(window.StatusBar) {
      StatusBar.styleDefault();
    }
  });

  $firebaseAuth().$onAuthStateChanged(function(user) {
      $rootScope.user = user;
  });
}])

auth-new.js

//https://github.com/EddyVerbruggen/cordova-plugin-googleplus

(function(firebase) {
'use strict';

    angular
        .module('App')
        .factory('Auth', Auth);

    Auth.$inject = ['$q', 'myConfig'];
    function Auth($q, myConfig) {
        
        var _nativeLogin = function(){
          return $q(function(resolve, reject) {
            window.plugins.googleplus.login(
              {
                'scopes': 'email profile',
                'webClientId': myConfig.googleClientId
                'offline': true,
              },
              function (obj) {
                resolve(obj)
              },
              function (msg) {
                reject(msg);
              }
            );
          });
        }
      
        return {
            login : function () {
                if(ionic.Platform.isWebView()){
                    return _nativeLogin().then(function (result) {
                        var credential = firebase.auth.GoogleAuthProvider.credential(result.idToken);
                        return firebase.auth().signInWithCredential(credential);
                    });
                }
                else{
                    var provider = new firebase.auth.GoogleAuthProvider();
                    provider.addScope('email');
                    provider.addScope('profile');
                    return firebase.auth().signInWithPopup(provider);
                }
            }
        };
    }
})(firebase);

auth-old.js

//Google scopes - https://developers.google.com/identity/protocols/googlescopes
//myConfig - Angular constants
(function(firebase) {
'use strict';

    angular
        .module('App')
        .factory('Auth', Auth);

    Auth.$inject = ['$cordovaOauth', 'myConfig'];
    function Auth($cordovaOauth, myConfig) {
        
        return {
            login : function () {
                if(ionic.Platform.isWebView()){
                    return $cordovaOauth.google(myConfig.googleClientId + '&include_profile=true', ["email", "profile"]).then(function (result) {
                        var credential = firebase.auth.GoogleAuthProvider.credential(result.id_token);
                        return firebase.auth().signInWithCredential(credential);
                    });
                }
                else{
                    var provider = new firebase.auth.GoogleAuthProvider();
                    provider.addScope('email');
                    provider.addScope('profile');
                    return firebase.auth().signInWithPopup(provider);
                }
            }
        };
    }
})(firebase);

Where were you 6 months ago!, haha great plugin thank you for your work!.

@alxanthony haha you're welcome!