If The Dark Pictures Anthology: House of Ashes generated the same gross revenue on the Epic Games Store, Supermassive Games would keep approximately $5,026,349 - that's $1,123,571 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $549,048. 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 $3,902,778.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.