You can try to a post from an earlier GibbsCAM version to 2021:
This is a free, user-configurable post-processor option available to GibbsCAM users that includes hundreds of basic templates for customization. Installation Procedures for 2021+ Versions
: This bundle, along with the marked-up G-code, is emailed to the post department or local reseller to initiate a service ticket. 4. Technical Considerations for 2021