If XCOM 2 generated the same gross revenue on the Epic Games Store, Firaxis Games, Feral Interactive (Mac), Feral Interactive (Linux) would keep approximately $42,912,038 - that's $6,941,551 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $5,715,278. 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 $34,039,121.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.