If 20 Minutes Till Dawn generated the same gross revenue on the Epic Games Store, flanne would keep approximately $2,687,822 - that's $645,236 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $230,158. 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 $2,042,585.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.