To add GTIN (EAN, MPN, UPC, ISBN) you have to use any custom field plugin. But you need our premium plugin to use custom field data for product feed. There is a popular free plugin called WC Field Factory. Using this plugin you can add GTIN or others information to your products both on frontend and backend. You have to make 2 group of the custom fields. One group for All type of products without variations so the fields will be available under General Tab and another is only for variations to add fields under Variations tab. After that, you have to map variations custom fields with other products custom field by making a Dynamic Attribute. Watch the video tutorial below to learn how to add GTIN (EAN, MPN, UPC, ISBN) to your products and use for product feed.