If Zombie Army Trilogy generated the same gross revenue on the Epic Games Store, Rebellion would keep approximately $18,056,819 - that's $3,269,757 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,325,930. 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 $14,267,924.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.