For ROAR to approve a body you need to be a ROAR Industry Affiliate member to be eligible to submit for approval, I believe the Affiliate membership costs $100.
Then the body approval procedure can be found in the application form here.
In Europe as there is no official Euro championship for WorldGT there is no European wide rules for the class, and no EFRA approved body list for WorldGT, which is a good thing as EFRA approval costs for a touring car body is €500 per body.
This means each country in Europe has their own rules for the class. In the UK the BRCA rules which are different to ROARs rules can be found here
and homologation is the same as 1/12th scale which means sending a sample body and reference photos to the committee who then make sure it is a reasonable replica of the original before approval.