Just a quick tip:
If you’re trying to use Google Oauth 2.0 to authenticate users in your MVC 5 application, start with this tutorial:
When you try to log in using a Google account, you’ll get an
access_denied error back from Google. To resolve the issue, read this blog post:
In a nutshell, you need to enable
Google+ API access for your application from the Google Developers Console.
More info in this Katana Project issue.