It appears the GTIN identifier field suddenly is not passing through to Google Merchant Center when using the Shopify Google app. All of my products that have GTIN identifiers are showing the message "GTIN is required for this product" when in fact my products do have the GTIN which I previously uploaded in the barcode field. Everything had been working fine for months. Is this a bug?
Interesting, make sure both brand and barcode are added and custom product is disabled.
Also make sure the barcode is officially stamped on the product packaging ie from the manufacturer and its only digits.