learn.getgrav.org
is a great place to start. It's our documentation. The best way to learn Grav, though, is to dive in and play with it. There are a handful of skeletons available, which act a lot like a RocketLauncher in that you get a complete site from the first install.
I actually bumped into an old paypal email and appears I was a RocketTheme member even before it existed:
Membership payment for JamboWorksTemplateClub, the email can't be a coincidence: This email address is being protected from spambots. You need JavaScript enabled to view it.