If Valheim generated the same gross revenue on the Epic Games Store, Iron Gate AB would keep approximately $160,713,137 - that's $17,719,376 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $21,779,064. 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 $127,744,541.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.