By the time licensing becomes relevant, your business is built around the platform and this gives Gumroad an unreasonable advantage in any negotiations. Imagine what they could say:

- You're now a competitor. Stop using our software (you can still sell on gumroad.com, hint hint)

- Give us 20% for 1 year (next year, who knows...)

- We won't give you a license, but we'll buy you out for next to nothing.

And at that point they can ask for anything because the alternative would be rewriting all your code.