If Warhammer 40,000: Rogue Trader generated the same gross revenue on the Epic Games Store, Owlcat Games would keep approximately $30,771,739 - that's $5,148,098 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $4,059,783. 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 $24,382,065.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.