Here is how Google Shopping Feed identify GTIN and MPN for a particular product
Google Shopping Feed by AdNabu automatically identifies the Barcode of a product provided in Shopify as GTIN. The SKU of a product in Shopify is mapped as the MPN in Google Merchant Center.
However, we also provide an option to override the predefined mapping of these fields using our Metafields feature.
It is also preferred to have valid GTIN for, e.g., 614141000012. In case you do not have a GTIN, it is better to leave it empty.