If Stronghold 3 Gold generated the same gross revenue on the Epic Games Store, Firefly Studios would keep approximately $4,086,314 - that's $931,291 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $420,861. 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 $3,155,022.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.