License key options for multisite setup

I have a multisite setup to sell sites on a subscription basis using WooCommerce on the main site (I use multiple payment gateways). I'd like to send new subscribers a license key they need to enter when activating a new site. I'll be using a template for setting up the new sites (a Gravity Forms app), but I haven't chosen the plugin yet. Any recommendations on how to implement the license feature? TIA.