If S.T.A.L.K.E.R.: Shadow of Chornobyl - Enhanced Edition generated the same gross revenue on the Epic Games Store, GSC Game World would keep approximately $2,737,041 - that's $655,304 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $236,869. 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 $2,081,737.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.