Hyde here from Shopify. Great question!
GTIN stands for Global Trade Item Number. GTINs are unique identifiers used internationally to store and locate product information. UPC, EAN, and ISBN numbers are examples of GTINs, which might vary in length depending on the product type. You can find the GTIN above or below the barcode on the packaging for your product.
As it says in this help document:
If you don't have a GTIN for a product, then you can request it from the manufacturer. Learn how to get a GTIN for a product you manufacture at the GS1 standards website. Don't invent false GTIN information for your products.
I hope this helps! Let me know if you have any other questions.
All the best, Hyde.