If Mortal Shell generated the same gross revenue on the Epic Games Store, Cold Symmetry would keep approximately $5,209,836 - that's $1,161,103 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $574,069. 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 $4,048,733.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.