If METAL GEAR SOLID V: GROUND ZEROES generated the same gross revenue on the Epic Games Store, KONAMI would keep approximately $7,886,825 - that's $1,708,669 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $939,112. 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 $6,178,156.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.