Hi,
You need to enter the column name of the custom product field in the GTIN setting of the plugin.
The GTIN needs to follow a certain set of rules:
support.google.com/merchants/answer/6324...en&ref_topic=6324338
It can't be a random number HikaShop would generate.
So usually, merchants would manually attribute a GTIN for each product.
Now, you can kind of automatically fill it. For example, you could use a mass action with a trigger "after a product is updated" with an action "update the values" on the column of your GTIN custom field and select the "operation" mode with something like this for example:
CONCAT('123456',LPAD(product.product_id, 8,'0'))
So in that example a product with the product ID 12345 would have the GTIN: 12345600012345
This mass action would run for a product when you save it, and if you want to manually run it for all your products, you can click on its "process" button.