We do not handle the app store, I only push to it. What I know is that for For Odoo15 (each new version) you need to purchase the module again. For the same version the module should still be valid.
Have you tried this link https://apps.odoo.com/apps/modules/14.0/bryntum_gantt_enterprise ?
or for the community version https://apps.odoo.com/apps/modules/14.0/bryntum_gantt/