Flesh out reference/gcp.html

[lindydonna]

https://pulumi.io/reference/gcp.html

[lindydonna]

@joeduffy @lukehoban This needs an owner.

[joeduffy]

@lindydonna Do you want to take an initial cut at this? It would be great to have more Google Cloud experts here on the team 😄

[lindydonna]

Sure thing. I can't promise M15.1 though, due to 2 conference talks next week.