That’s strange – it would be good to ask if they’re compatible with plugins/themes using Easy Digital Downloads Software Licensing. It’s a really popular service to use with premium WP products.
Update on MainWP being not able to detect new versions of GP Premium:
The people of MainWP looked into it and found a solution. The latest versions of MainWP are now able to detect new versions of GP Premium. Makes MainWP very useful for us.