If Red Dead Redemption generated the same gross revenue on the Epic Games Store, Rockstar Games would keep approximately $11,188,313 - that's $2,255,092 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,389,315. 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 $8,804,340.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.