If Amnesia: Rebirth generated the same gross revenue on the Epic Games Store, Frictional Games would keep approximately $4,960,590 - that's $1,110,121 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $540,080. 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 $3,850,469.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.