If Legend of Heroes: Three Kingdoms generated the same gross revenue on the Epic Games Store, Free Wing would keep approximately $2,597,634 - that's $626,789 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $217,859. 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 $1,970,845.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.