Their "gross revenue" is $5m a month. Their part of that is 350 000$ a month (which is still their gross revenue, but I feel like that's what you meant).
> So I built Gumroad. Fast-forward over seven years and we're doing about $350,000 in revenue monthly, helping creators earn over $5,000,000 a month.
The $5m they process is not gross revenue to Gum Road since they're not the beneficial or legal owners of the $5m in payments received. Their X% share of that $5m is their gross revenue.