If The Vanishing of Ethan Carter generated the same gross revenue on the Epic Games Store, The Astronauts would keep approximately $2,805,754 - that's $669,359 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $246,239. 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 $2,136,396.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.