Some e-commerce platforms or online marketplaces (such as e.g. Amazon) might require that the products you sell on its marketplace have a Global Trade Item Number or GTIN. However, if you sell products through Gelato, they won't have a GTIN. In such cases, you might have to request a GTIN exemption from the e-commerce platform or online marketplace.
Please refer to their website, Help Center and Customer Support for additional information.