Email support@itch.io, tell them the following information:
Your e-mail you registered your account with
The product you bought
The e-mail you used for the purchase
The date of the purchase
If you got charged for the purchase
I think that should be enough to allow them to sort it out
In addition you could ask the author of Gutty Tiny: Clean City Game Assets to look up your receipt number, as having that may be of use to the support staff.