If X4: Foundations generated the same gross revenue on the Epic Games Store, Egosoft would keep approximately $25,034,869 - that's $4,300,606 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $3,277,482. 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 $19,818,646.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.