Dropbox and Instagram both have much stronger network effects.

well, if Gumroad is first of its kind, or the biggest one, and will not turn out a rip-off, don't you think that people who recommend it would be a "network effect" ?

Network effect means that the more people use a service, the more useful it becomes. That doesn't seem to be the case here.

But there is the built-in marketing aspect since every user must send a lot of people around them to Gumroad in order to benefit from it.