Connecting Gravity Form with Pro Sites

I need help with automatically setting up Pro Sites billing for a new blog user using a Gravity Forms form.

I am creating a hosting service using Multisite, Gravity Forms, Blog Templates and Pro Sites. I want the user to:
- register a user account in WP through a Gravity Form
- choose a blog template / choose a Pro Site level
- pay for their service
- have the site generated automatically
- link to Stripe and PayPal
- have the Pro Site billing setup (through the Gravity Form)

I believe I've identified all the pieces except the last part, setting up the Pro Sites billing. Any help would be appreciated.