If Heroes of Might & Magic III - HD Edition generated the same gross revenue on the Epic Games Store, Dotemu would keep approximately $7,502,364 - that's $1,630,029 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $886,686. 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 $5,872,335.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.