Pay What You Want Pricing
To allow your customers to name their own price for your product, simply enter "0+" as the price. Your customers can enter "0" to pay nothing, but if they choose to pay, they have to pay at least $0.99 (USD) as a donation.
If you'd like to set a lowest possible donation price, simply enter that number followed by "+"
Suggest a Price
Sometimes it's emotionally taxing to set a price on your hard work, but Gumroad allows you to be coy with your customers. In the Edit tab of your product, once you have added your product, you can also set a "suggested" price that Customers will see when entering their donation amount. Change this by clicking the price box, then Save changes to make these settings go live.
If a product is sold for free, as long as any contact information is recorded during the transaction (a name or email address) it will be recorded as a sale in your Customers tab as $0.
You can learn how to set your product to be totally, completely free here.