FEATURE: GTIN is Now Available for Variant Inventory

This field will be sent in the Google Base feed when present, otherwise if they use variant inventory, it will fall back on the overall GTIN for the product if there is one.

Additionally, this field is now supported in variant inventory import/export, and there is also a new control merge available on the product details page,

$$VARIANTGTIN$$

This merge can be optionally placed on the product details page layout, and it will automatically update based on the selected variants of a product, in the same manner as the price.