I have many items that are not publishing to Google Products. The error is missing MPN or GTIN. There is a GTIN (UPC) set under barcode. All are marked as not custom products, and I don't see a difference in setup from a product that publishes and one that doesn't. Is there another place to check?
Use the identifier_exists attribute: https://support.google.com/merchants/answer/6324478?hl=en