If POSTAL: Brain Damaged generated the same gross revenue on the Epic Games Store, Hyperstrange would keep approximately $3,167,703 - that's $743,394 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $295,596. 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 $2,424,309.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.