To make your product reviews to your products, Google will use GTIN and metadata information such as SKU, Brand + MPN pairs, and product URLs. To do this, you need to:

*Note: 

  • Shopify's Google Shopping app does not allow you to use SKU as item id in your products feed. You can only use the Shopify Product ID.
  • If you change the product ID in your product feed inside your Google Merchant Center account, you will lose the product's advertising history in Merchant Center and AdWords. Please study Google's documentation for product ids for further details.