If The Witcher: Enhanced Edition Director's Cut generated the same gross revenue on the Epic Games Store, CD PROJEKT RED would keep approximately $19,094,349 - that's $3,423,029 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,467,411. 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 $15,093,232.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.