If HELLDIVERS™ 2 generated the same gross revenue on the Epic Games Store, Arrowhead Game Studios would keep approximately $679,765,038 - that's $64,905,913 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $92,558,869. 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 $540,626,735.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.