In short
You need one whenever the product has one. Google's product data specification requires the GTIN for items that carry a manufacturer-assigned identifier, and submitting an item without it, or with a wrong one, leads to disapprovals or limited eligibility. Products that genuinely have no GTIN, such as handmade, custom, vintage or own-brand goods, are handled by supplying brand and MPN and declaring that no identifier exists. Guessing, reusing or inventing codes is the outcome to avoid.
The rule, and why Google cares about it
A GTIN is the globally unique number assigned to a product by the party that brings it to market, and it appears as a UPC in North America, an EAN in Europe, a JAN in Japan or an ISBN on a book. When a manufacturer has assigned one, Google expects it in the feed.
The reason is matching. The same product is sold by many retailers, and Google needs to know that your offer and twenty others describe one item so it can build a product record, show comparisons, present a single product page and reason about price. The identifier is what makes that possible without guessing from titles.
What that means for a retailer is straightforward: an item with a correct GTIN joins a product that already exists in Google's understanding, complete with whatever information has accumulated around it. An item without one is a standalone offer that has to be understood from your own data alone, which reduces how often it can be matched to a specific request.
That matching effect now extends beyond Shopping results into the product panels in AI answers, which is why identifier coverage has become a more consequential piece of feed hygiene than it used to be.
Products that genuinely have no identifier
Plenty of items have no GTIN and never will. Handmade goods, made to order furniture, custom jewellery, art, vintage and secondhand items, bundles you assemble yourself, and own-brand products from a manufacturer that never registered codes all fall into this group.
For these, supply the brand and the MPN, and set the attribute that declares no identifier exists. That combination tells Google the omission is deliberate rather than an error. Do not leave the field blank without the declaration, because the system cannot distinguish between a missing value and an absent one.
The MPN should be your own internal part number if you are the manufacturer, and it should be stable. Changing it between uploads breaks the continuity of the item's history.
If you manufacture your own products at any volume, it is worth obtaining a company prefix from the standards body and assigning proper GTINs. It costs something and takes administration, but it makes your products identifiable to every retail system, not just Google, which matters as soon as you sell through marketplaces or wholesale.
Bundles need care. A multipack that the manufacturer has assigned its own code to uses that code. A bundle you assembled from separate products has no GTIN of its own and should be declared accordingly, with the multipack and bundle attributes set correctly.
Keeping identifiers correct at catalogue scale
Source them from the supplier rather than from a lookup site, and store them in a dedicated field rather than in a description or a title. Suppliers usually provide them in their product data, and the ones that do not can normally be asked.
Validate them before submitting. A GTIN has a check digit, so a simple validation rule catches transposed and truncated numbers before Google does. Watch particularly for codes stored in a spreadsheet where a leading zero has been stripped or the number has been converted to scientific notation, which is one of the most common causes of a whole feed failing.
One identifier belongs to one product variant. Every size and colour of a garment has its own code, and reusing a single code across a range is a data error that produces mismatches. The item group ID is what ties variants together, not a shared GTIN.
Do not put a UPC in the MPN field or the other way round; they are different attributes with different meanings. And never submit a code taken from a similar product to fill a gap. It will be matched to that other product, your listing will be attached to the wrong record, and correcting it afterwards is slower than doing it properly.
Audit coverage quarterly: what share of your catalogue has a valid identifier, which suppliers are the gaps concentrated in, and which of those gaps are genuine rather than administrative.
What goes wrong when identifiers are missing or incorrect
The visible symptom is disapproval, with a message naming a missing or invalid identifier. Those items stop serving in paid campaigns and drop out of free listings, so the effect on revenue is immediate and category-wide if the gap follows a supplier.
The less visible symptom is limited eligibility. Items can remain live while being harder to match to specific queries, which shows up as low impression share on products that look fine in the interface. Stores often spend weeks adjusting bids on a problem that was a data gap.
A wrong code is worse than a missing one, because the offer attaches to the wrong product record. Price comparisons then look strange, the specifications shown alongside your listing describe a different item, and shoppers who click arrive at something other than what they expected.
The fix sequence is the same each time. Pull the disapproval list grouped by issue, trace the affected items back to their source, correct the data at that source rather than in a patch file, and confirm the corrected values appear in the next fetch. Where the source cannot be corrected quickly, a supplemental feed is the right temporary place for overrides, kept as a short-lived measure rather than a permanent second copy of the truth.
Related questions
What happens if a wrong GTIN is submitted?
The item can be disapproved for an invalid value, or matched to a different product entirely, which is the more damaging outcome because everything shown alongside your offer then describes something else. Validate the check digit before upload and treat any code you cannot trace to the supplier or the manufacturer as unverified.
Do handmade or custom products need identifiers?
No, provided you declare that none exists and supply brand and MPN instead. That declaration is what separates a legitimate absence from an omission. If you make products in volume and sell through other channels, obtaining real codes is worth considering, since it makes the catalogue usable across retail systems generally.
Is a UPC the same thing as a GTIN?
A UPC is one form of GTIN, the twelve-digit version common in Canada and the United States, while EAN, JAN and ISBN are other forms. Google accepts them in the same attribute, so the practical answer is to submit whichever code the manufacturer assigned, without padding or reformatting it.