If Forza Motorsport generated the same gross revenue on the Epic Games Store, Turn 10 would keep approximately $17,062,796 - that's $3,122,913 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,190,381. 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 $13,477,224.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.