Fix “Invalid value [gtin]” on Google Merchant Center

Invalid GTIN is the close cousin of Missing GTIN, and the fixes overlap heavily. The difference: missing GTIN means the field is empty; invalid GTIN means the field has a value but the value doesn't validate. The four most common causes: check digit wrong (the last digit is a calculated checksum — bad data fails), formatting issues (dashes, spaces, leading zero strip), wrong field type (Excel strips leading zeros from numeric fields), placeholder values (1234567890123 doesn't pass validation).

The Excel-strips-leading-zeros problem deserves specific attention. A valid GTIN starting with 0 (like `0123456000018`) opens in Excel as a number and becomes `123456000018` — instantly invalid. Any CSV passing through Excel between source and feed risks this. The fix is to enforce text format on the GTIN column at every step, or skip Excel entirely.

Beyond the formatting issues, the validation Google runs has two layers. First, the check-digit algorithm verifies the GTIN is structurally valid. Second, validation against GS1's database checks whether the GTIN is actually registered to a manufacturer. The first catches data quality issues; the second catches fake GTINs that pass structural validation but aren't real. The two-layer validation is why placeholder GTINs (0000000000000) fail both — they're structurally invalid AND not registered.

The AI Shopping Feeds Audit feature validates GTINs across the full catalog in seconds, flagging structural issues, check digit failures, and unregistered identifiers separately. The Optimiser auto-corrects formatting issues at feed generation, preserving leading zeros and stripping non-digit characters.