This subject has been partially treated in oprevious post (https://multivendorx.com/support-forum/topic/how-to-add-gtin-for-variations/)
but:
1/ GTIN per main product does not make sense as GTIN is a “per variation” code.
2/ The plugin you indictaed in Github (https://github.com/wcmarketplace/dc-woocommerce-multi-vendor/issues/863) (https://wordpress.org/plugins/product-gtin-ean-upc-isbn-for-woocommerce/) is obsolete, not maintained and not supported ! (
3/ GTIN/MPN is a mandatory Field for Products feeds like Google Merchant.
So please consider fixing this problem.
Thanks,