Hikashop Google Products Plugin - GTIN

Hi,

If you hover on the label of the GTIN field of that plugin, it says:

If you enter here the column name of a custom product field, it will use the value of the custom field for each product allowing you to have a GTIN for each products for which the custom field is filled. If you leave this field empty, no GTIN will be added to the feed.

So follow the indications there and create a custom product field via the menu Display>Custom fields, enter the column name of that field in that setting, and then fill in your products GTIN in that custom field in each product edition page and that will do it.