If Galactic Civilizations III generated the same gross revenue on the Epic Games Store, Stardock Entertainment would keep approximately $7,185,889 - that's $1,565,296 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $843,530. 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 $5,620,594.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.