If Gothic II: Gold Edition generated the same gross revenue on the Epic Games Store, Piranha Bytes would keep approximately $4,836,113 - that's $1,084,659 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $523,106. 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 $3,751,453.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.