GTIN is a unique identification number for products. How can I solve errors related to GTIN?
AdNabu's Google Shopping Feed app automatically maps GTIN from Shopify to Google Merchant Center. AdNabu uses Barcode in Shopify as GTIN in Google Shopping. If your barcode is not properly formatted or is longer/shorter than Google's requirements, You will see an error in Google Merchant Center.
The only way to fix this error is to change the Barcode field in Shopify.
Here is an article for further reading on GTIN