If Disco Elysium - The Final Cut generated the same gross revenue on the Epic Games Store, ZA/UM would keep approximately $60,610,976 - that's $8,619,180 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $8,128,769. 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 $48,117,821.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.