If Six Days in Fallujah generated the same gross revenue on the Epic Games Store, Highwire Games would keep approximately $10,969,478 - that's $2,222,764 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,359,474. 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 $8,630,266.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.