I just happen to come across two plugins that have an option in multisite right in the network admin to apply the license to all current and future subsites (BB Powerpack & UABB):
I’ll definitely look into it – I agree it would be nice.
However, it’s super easy to do by default. Just activate GPP on one of the subsites, and use that subsite to perform the update. Updating will update the plugin for all of the sites on the network.