I would say skip trying to hunt through the Chem Cookies where Skunkmaster Flex found GMO. Skunkmaster’s said that GMO was a complete outlier from everything else in the pack of Chem Cookies and thinks it might have been a random seed from a different cross.
Hoku seeds did an F2 of Donny Burger within the past few year or so which is the first back cross of GMO that Skunkmaster did but it looks like it’s sold out. S1s or back crosses of GMO would be your best bet but unfortunately I’m not aware of any in stock. You could reach out to Hoku and see if they have any packs left.