I'm not HR but I'm pretty sure the anti-overtime provision is completely illegal at least in countries like the US and Canada. I would outlaw overtime instead of pay people less for overtime.
You can't just voluntarily work more hours than your contract states and expect to get paid more.
This is incorrect. The tldr is that US employees are either "exempt" or "non-exempt". Without getting into pedantic details, employees can be categorized as exempt if they make over about $23k/year. Exempt employees are not entitled to overtime at all, and the Fair Labor Standards Act doesn't prohibit reducing someone's hourly pay for overtime in the unusual way that Gumroad does.
If you are a contractor you should be clear to do whatever you want with regards to overtime (because it doesn't exist.) Only employees are regulated by overtime laws. However as many know the definition of contractor can be rebutted by an aggressive state DOL.
Then it comes down to control: Who sets the schedule. Who pays the expenses. Who decides what is worked on. Who decides how long someone is working, etc.
Remote can only help Gum's case, and the way the founder describes the arrangement, it would seem to pass muster.
Be warned that passing muster does not mean that legally he would not need to fight it. Founder may mean expensive lawyer fees to defend Gum. So word of advice for Gum - document your processes for task management and scheduling internally , just as you document your code!
Be clear that it is the professionals (not you) who are dictating what gets done, and when. That should be enough.