Technically speaking, the GTIN is just another attribute in terms of the formatting of the JSON-LD structured data. However, search engines understand the value of the GTIN, as it allows them to connect to apps that scan barcodes and search for relevant results. In terms of being a 'key' (that is, a unique identifier for a product), the value of the GTIN becomes important to a search engine indexer, which can use its uniqueness to understand that it is seeing the exact same product on multiple websites.