You can even use a different email address when purchasing :).
When the RPG Playground website sends you to the product page on gumroad, I send a hidden field with your RPG Playground email address. Once you pay, gumroad will send back that hidden field with the email address.
On my side, I first try if there’s a hidden field, and link that user. If there was no hidden field (maybe people went straight to the product page), I try the email address of the one who purchased. If that email is also not linked to a user, you’ll just have to email me and I’ll link it manually ;).