How to add Google Plus authorization to the site?

Question

Google

Greetings. I need to add Google Plus authorization to the site with a pop-up, but I can’t find a normal example where this is done in js (I was looking for about 15-20 hours).
I would be grateful for any instructions or code example on how to do this. It should start like this - There must be a custom class. Thanks for the help!
<a class="some-class" onclick="gp_auth()">Вход</a>

Answer 1

Isn't it? https://developers.google.com/+/web/signin/