If Gotham Knights generated the same gross revenue on the Epic Games Store, Warner Bros. Games Montréal, QLOC would keep approximately $28,538,177 - that's $4,818,140 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $3,755,206. 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 $22,605,368.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.