I’ve found how to change this for all my products via Google merchant centre but I have some products with barcodes and others without, such as hand made products by small suppliers. How do I change the identifier_exists value to no for some of my products?
In Google Merchant Center go to Products > Feeds > click on your feed > feed rules.
Here setup a if else. For example if gtin has no value set identifier exists to no.
More info: https://support.google.com/merchants/answer/7450276?hl=en