Hi,
I encounter the same issue. I am not totally sure I understand but I will try again.
However, I would like to make sure of one thing:
I am an illustrator and I use Print On Demand companies within my Shopify store.
Therefore, is it not these companies that should know/provide the gtin/mpn? How can I know which gtin/mpn to use?
I also chose the option of “custom product” thinking I can skip this process but through your answer I understood that this is not correct.
It would be great if I can learn exactly how to give a gtin/mpn in each product and get approved by google. Without it, it is almost meaningless to have a store.
Looking forward to your reply,
Kind regards,
Vasiliki, rocketsack.com