If War for the Overworld generated the same gross revenue on the Epic Games Store, Brightrock Games would keep approximately $3,947,591 - that's $902,916 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $401,944. 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 $3,044,675.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.