Donation rewards are outlined here

To claim the rewards, please go to settings -> security and add the email you used on gumroad to your account then click "claim donation rewards", thank you 💖😻

Note: we don't see any of your payment details, it's all managed by gumroad and we can only see your email, however, if you're still concerned about privacy you can use paypal or make a virtual prepaid card using https://privacy.com