If Atomic Heart generated the same gross revenue on the Epic Games Store, Mundfish would keep approximately $43,946,381 - that's $7,094,352 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $5,856,325. 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 $34,861,894.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.