GTIN/Barcode is a required field only if you are the manufacturer of those products. However, products submitted without any unique product identifiers are difficult to classify and may not be eligible for all Shopping programs or features.
This means that products with no GTIN would have a lower ranking and potentially lower visibility, specifically through Google's Free Listings. However, you can still publish them to the Merchant Account Center with a warning tag.
For more information on GTIN from Google, click here.
If your products have invalid GTIN, we will not be able to publish them to Google and will need you to update your GTIN/Barcode information in your Shopify Admin Panel.
Meanwhile, for a quick Go-to-Market, you can use the "LEAVE BLANK" function to remove the GTIN information from your products and publish them to your Merchant Center. You can always revert them back to the original value when the new GTIN information is added to your product catalog in Shopify.