If Warhammer 40,000: Space Marine 2 generated the same gross revenue on the Epic Games Store, Saber Interactive would keep approximately $183,188,499 - that's $19,762,591 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $24,843,886. 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 $145,622,670.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.