You open Google Merchant Center and a chunk of your catalog carries the warning “Limited performance due to missing identifiers [gtin]” — or worse, products are disapproved with “Invalid value [gtin]”. Here's what Google actually wants, and the fastest way to satisfy it.
What a GTIN is (and why Google cares)
A GTIN is the number under a product's barcode — EAN in Europe (13 digits), UPC in North America (12 digits), ISBN for books. Google uses it to match your offer to its product catalog: matched products get richer listings, better placement in comparison results, and generally cheaper clicks. Products without one still run, but with limited performance — which is exactly what the warning says.
Step 1: find the barcodes you already have
If you resell branded goods, the numbers exist — they're on the packaging, in the supplier's price list, or in the invoice line items. In Shopify, the field is called Barcode (ISBN, UPC, GTIN, etc.) and lives on each variant. Fill it there, not in the feed: your store stays the single source of truth, and every channel gets the fix at once.
Bulk-editing tip: in Shopify admin, Products → select all → Bulk edit → add the Barcode column, and paste from your supplier's spreadsheet. A thousand products is an afternoon, not a week.
Step 2: watch out for fake or reused numbers
Don't invent numbers, don't reuse one GTIN across variants, and don't copy a competitor's. Google validates check digits and cross-references its catalog; a wrong GTIN produces “Incorrect identifier [gtin]” disapprovals that are worse than having none. Each variant (size, color) has its own barcode.
Step 3: products that genuinely have no GTIN
Handmade goods, custom products, house brands, vintage items — these legitimately have no barcode. Google accepts that, but only if the feed says so explicitly: the product must carry identifier_exists = false (and then brand + MPN aren't required either). Leaving the GTIN field silently empty is what triggers the warning; declaring “this product has no identifier” is a valid, fully-approved state.
How Feedrobin handles it
Feedrobin reads barcodes straight from your store and checks them before Google sees the feed. Products without one are counted on your dashboard with a one-sentence fix, and if they're genuinely barcode-less you can mark them as such — the feed then carries the correct identifier_exists flag automatically, for every channel at once.