Your proof of income has to come from Gumroad, they are your customer. As the Gumroad details remind you the "customers" are buying from Gumroad, not you. In order for Gumroad to sell it to them, they have to "buy" it from you.
Exactly what the payments are, and how you ""prove" them will vary by contract and local laws. It could even be considered "royalty" payments, similar the an author being paid for sales of a book, since that's what it looks like in what little information given in the question.
Finally, since Gumroad is located in the USA, they have to comply with American tax laws, which should include the requirement that they supply the proper "income" information to their providers/sellers (you). Again, depending on the contract you have with them the forms provided will differ. It could be a simple payment distribution statement or a Form 1099 from the American IRS standards.
To find out exactly what, and when, they provide documentation, and how to get it, contact them directly. Go to their support page and type "income Verification" in the search box. That will trigger a "Contact" form. Fill that out with your information and ask the question, stating what you need, and why. Click the "Contact Support" button.