I can't talk to gumroad specifically but I run a shopify store that offers both shopify's built in payment CC processor and paypal as payment options. 56% of people checkout with paypal vs pulling out their credit card.
My guess is if you have a paypal account with your credit card on file it's much easier to just checkout with paypal than manually type in your card info.
My guess is if you have a paypal account with your credit card on file it's much easier to just checkout with paypal than manually type in your card info.