GTIN is just a big scary initialization for the 12 digit UPC barcode that you see on almost everything you buy from Apples to Zip Lock Baggies. When you are in your favorite retailer buying books, collectibles, home furnishings, whatever the barcode they scan is the GTIN. There are some barcodes that are not 12 digits these are not UPCs and can not be used to Universally Identify your products.
If you have ever posted an item on eBay and you entered the UPC number you found on the back of the box right below the barcode? If so then as long as your images do not contain any text or water marking, then you listing should be on Google Shopping.