Weirdly, I feel like it's hard to get organic traffic / supporters/ followrs from Gumroad itself, whereas Itch's community spirit and interactive nature fosters more engagement and, if nothing else, lots of organic traffic on your page. Maybe there's another dimension to GR that I'm not privy to, but it seems more like a "Here you go, you're on your own!" type of experience and Itch is way more warm and inviting in comparison.
I also found this thread on a Google search, I like that it's not even that old yet. Great minds think alike!