If Pillars of Eternity II: Deadfire generated the same gross revenue on the Epic Games Store, Obsidian Entertainment would keep approximately $17,207,923 - that's $3,144,352 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,210,171. 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 $13,592,666.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.