If Red Dead Redemption generated the same gross revenue on the Epic Games Store, Rockstar Games would keep approximately $11,802,034 - that's $2,345,755 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,473,005. 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 $9,292,527.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.