If Red Dead Redemption 2 generated the same gross revenue on the Epic Games Store, Rockstar Games would keep approximately $551,875,622 - that's $53,279,602 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $75,119,403. 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 $438,896,517.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.