I think Sahil addressed this elsewhere before, but my own assumption is that if someone is willing to hand over money for something they understand that creator

I think Sahil addressed this elsewhere before, but my own assumption is that if someone is willing to hand over money for something they understand that creators deserve to be paid for their work and they won't then go ahead and distribute it at no cost. The latter shouldn't be a concern anyway, even if the link was unsharable someone could still download and re-upload elsewhere. The technical solution I assume (which actually goes against what appears to be the idea of Gumroad -- simple for everyone) would be a Paypal IPN type system. You can upload directly to Gumroad and that has no issue of unauthorised sharing, but if you use the pay wall for a link type then you do risk it.