If Thank Goodness You're Here! generated the same gross revenue on the Epic Games Store, Coal Supper would keep approximately $2,856,763 - that's $679,792 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $253,195. 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 $2,176,971.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.