I decided to use social media benefits on my page and currently I'm implementing Google+ Sign-In. 
One of the pages on my website should be accessible for logged in users only (adding stuff to the page). I am logging user to website via JavaScript.
I'm aware that javascript is executed on client-side but I am curious is it possible to restrict access to the certain page using only javascript.
You cannot perform reliable access control using only client-side javascript.
This is because since the javascript is executed on the user's browser, the user will be able to bypass any access control rule you've set there.
You must perform your access control on server-side, in your case in Python code.
Generally, people also perform some kind of access control check on the client side, not to prevent access, but for example to hide/disable buttons that the user cannot use.
Google+ Sign-In - Page accessible for logged in users only