This depends upon the tier of product you have purchased.

If you have purchased a licence for a Creator tier of GridForge Studio (ie. Creater, Creator Premium) then yes, you have full commercial rights to sell any static, exported content (like PDFs for KDP, images, etc.) that you create with the software.

Educator versions do not have commercial rights, material produced using Educator versions may only be used for personal or educational purposes. Any material created using GridForge Studio Educator versions may not be sold