Oh yep, they must have ended the deal without updating the site perhaps. I actually didn't know it no longer worked until you commented. I have updated information on my end to reflect the change. I'll talk to the Playtiles team and notify them that there is some confusion around the GUMPY20 code. Thanks for bringing it to my attention.
EDIT: I have spoken to the Playtiles Team and they will renew the GUMPY20 promo code for a period of time. I'll update the window of time it will be active once I receive that info from Playtiles.