A GTIN should always be traceable back to the brand owner. If you buy a GTIN from a eslewhere (also known as resellers) this will never be the case as the number will either be made up or registered to another company. This means it will never be associated with your brand or product correctly. If you want to sell your products in retail stores, the GTIN will be checked and your products will be rejected as the GTIN will not be registered to you. If you have the product listed in Amazon's brand registry then yes, you can update this. And on eBay, it can be updated at any time too.