If Forza Horizon 5 generated the same gross revenue on the Epic Games Store, Playground Games would keep approximately $222,009,515 - that's $23,291,774 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $30,137,661. 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 $176,503,023.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.