Google's gtin page says products with a missing GTIN may have limited visibility. Add the GTIN the manufacturer assigned to each product and each variant. For products without the identifiers their category requires, Google's product data specification says to set identifier_exists to no.
What Merchant Center reports, and what a GTIN is
Merchant Center reports a missing GTIN as “Limited performance due to missing value: GTIN [gtin]”, and a GTIN value that is not valid as “Invalid value [gtin]”.
A GTIN is a unique numerical identifier for commercial products, usually associated with a barcode printed on retail merchandise — for example EAN in Europe (13 digits), UPC in North America (12 digits) and ISBN for books. Google uses it to understand your products and match them to search queries. Google's gtin page says products with a missing GTIN may have limited visibility, and that a product that has a GTIN assigned but is submitted without it could be disapproved.
How to fix it
Step 1: add the GTINs the manufacturer assigned
Google's gtin page recommends the attribute for all products with a GTIN assigned by the manufacturer. Shopify's page Syncing your products says to open the product in the Shopify admin and add the barcode (GTIN) in the Inventory section; for WooCommerce, see the WooCommerce GTIN field guide.
Step 2: watch out for fake or reused numbers
Use only the GTIN the manufacturer assigned. Google's gtin page says each product and each variant (different colors or sizes) has its own GTIN, not to guess or make up a value, and that the check digit must be present and correct. Google's page How to fix: Incorrect GTIN says that issue tends to occur when the GTIN of another product is re-used.
Step 3: declare the products that have no unique product identifiers
Google's identifier_exists page lists products that might not have unique product identifiers: custom or one-of-a-kind goods, like custom T-shirts, art or handmade goods, and products made before GTINs were introduced, like vintage products, antiques and books published before 1970. Google's product data specification says to set identifier_exists to no for a media item without a GTIN, for an apparel item without a brand, and, in all other categories, for a product that has neither a GTIN nor both an MPN and a brand; the value can be no or false. The specification also says to still submit the identifiers the product has, and that a product that does have unique product identifiers may be disapproved if it is submitted with no.
How to check it is fixed
Google's page How to fix: Limited performance due to missing value: GTIN [gtin] says: “After making the requested changes, check that you’ve fixed the issue by making sure it’s no longer listed on the ‘Needs attention’ page.” It also says: “It may take some time for your change to be reflected on the ‘Needs attention’ page.”
How FeedRobin handles it
FeedRobin reads barcodes from your store: the variant barcode on Shopify, the GTIN field in WooCommerce. When it builds feeds from a connected Shopify store or from a feed URL, it leaves out a code whose length or check digit is wrong. The free feed check counts products without a GTIN on any feed URL, with no account; for a Shopify store address it does not check barcodes. In the Google, Meta, Microsoft and custom feeds, a product that has neither a GTIN nor both a brand and an MPN carries identifier_exists = no when it comes from a feed URL. A product from a Shopify store carries no identifier_exists.
Sources
Google's attribute pages for gtin and identifier_exists, its pages About unique product identifiers, How to fix: Limited performance due to missing value: GTIN [gtin], How to fix: Invalid value [gtin] and How to fix: Incorrect GTIN, its product data specification, and Shopify's page Syncing your products.