If Sid Meier's Civilization VII generated the same gross revenue on the Epic Games Store, Firaxis Games would keep approximately $75,137,881 - that's $9,939,807 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $10,109,711. 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 $59,673,314.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.