If Grand Theft Auto IV: The Complete Edition generated the same gross revenue on the Epic Games Store, Rockstar North would keep approximately $50,947,185 - that's $7,740,653 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $6,810,980. 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 $40,430,715.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.