What is the GTIN requirement?

Some major search engines ask for 2 unique product identifiers to satisfy the GTIN requirement for brand name products sold by multiple merchants. Google Product Feeds must have GTINs, and tests the accuracy of the data provided. You may see errors or warnings about this in your Google Merchant Center account when viewing your product feeds. 

The GTIN requirement is satisfied with 2 of these unique product identifiers which include UPC codes, Manufacturer Part Numbers (MPNs), International Standard Book Numbers (ISBNs) and brand names.