Google+ vs Google Identity Platform

tl;dr: can someone explain what exactly is the difference in implementing client-side Google login flow between these two platforms?

The backstory:

I've been trying to implement client-side Google sign in to my website. First, I implemented the Google+ platform with global settings using tags, so user session is monitored. Got the info here: http://ift.tt/15SYG6P

However, I encountered a problem where the site would automatically check for user login state if the user is not logged in, which resulted in many 'toastr' messages of 'Logged out', which I implemented in the signInCallback function. It was pretty annoyting.

So I did some research and stumbled across their 'quick start app' and browsed through it. It is WAY more complicated than their guide, many elements were documented on Google Identity Platform, here: http://ift.tt/1Fe6jpu

Now I don't really understand what is the correct way of implementing their login - is it the lightweight Google+ button with tag callback check for user state, or is it the robust GIP way with listeners, gapi instances and all? What exactly different do these platforms offer?