If Rhythm Doctor generated the same gross revenue on the Epic Games Store, 7th Beat Games would keep approximately $8,795,693 - that's $1,894,574 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,063,049. Epic charges 0% on the first $1 million in annual per-app revenue and then 12% on the remaining amount. On GOG (30% cut), the developer would earn approximately $6,901,119.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.