If Stronghold 2: Steam Edition generated the same gross revenue on the Epic Games Store, Firefly Studios would keep approximately $1,499,866 - that's $402,245 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $68,164. 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 $1,097,621.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.