If The Dark Pictures Anthology: The Devil in Me generated the same gross revenue on the Epic Games Store, Supermassive Games would keep approximately $5,240,089 - that's $1,167,291 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $578,194. 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 $4,072,798.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.