I am currently having that same issue. Didi you ever get it resolved?
Topic summary
Widespread “Limited performance due to missing value [gtin]” notices in Google Merchant Center are linked to a Shopify Google channel bug where barcodes aren’t sent via the Content API unless a specific metafield is set.
Key guidance and rules:
- If a manufacturer assigns a barcode: submit brand + GTIN (+ MPN if available) and set identifier_exists = yes. If no barcode exists: remove brand/MPN/GTIN and set identifier_exists = no. Some categories/countries (since June 2021) require GTINs—obtain GS1 codes if needed.
- Common mistake: using SKU (internal stock code) as MPN (manufacturer part number) is incorrect.
Shopify-specific fixes/workarounds:
- Set the Google “custom product” metafield to false for products with barcodes to ensure GTIN is passed. Do this per product or in bulk via apps (e.g., Metafields Guru, Matrixify) or use a supplemental feed/feed rules. The standard Shopify CSV “Google Shopping / Custom Product” field doesn’t control this. Changes typically reflect in ~30 minutes.
- Consider third‑party feed apps (Multiple Google Shopping Feeds, DataFeedWatch). Avoid disabling automatic updates to prevent other issues.
Status/outcomes:
- Multiple users confirm the workaround resolves flags. Shopify support awareness is inconsistent; no confirmed permanent fix announced. Issue remains open, with scalability concerns for large catalogs.