If The Slormancer generated the same gross revenue on the Epic Games Store, Slormite Studios would keep approximately $3,016,298 - that's $712,425 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $274,950. 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 $2,303,874.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.