Hi Jessi,
You can set zero GST for orders shipping to outside India. Our extension will automatically calculate CGST, SGST and IGST based on the shipping address provided by the customer on the checkout page. You have to set the production state in the backend in order to achieve this automatic split of GST.
To answer your queries on Vendor related GST features, please let me know the marketplace extension used.Please mail us to support@modulebazaar.com
Thank you