Protect pages with Google OAuth?

I’ve been looking into Jekyll for building a documentation site for my employer, and while I like the basic concept I’m thinking Kirby would be a better fit. One requirement is that we’ll need to restrict certain areas or pages based on Google Authentication (we use Google apps) and since Kirby is just PHP I imagine it won’t be too hard.

I’ve seen that there is a plugin for Twitter and GitHub OAuth, but since I need Google I guess I’ll have to roll my own. I’m mostly a client-side guy so I’m wondering if anyone can point me in the right direction or give some pointers?