If This War of Mine generated the same gross revenue on the Epic Games Store, 11 bit studios would keep approximately $42,928,448 - that's $6,943,975 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $5,717,516. 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 $34,052,175.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.