If Shadow of the Tomb Raider: Definitive Edition generated the same gross revenue on the Epic Games Store, Eidos-Montréal, Crystal Dynamics, Nixxes, Feral Interactive (Mac), Feral interactive (Linux) would keep approximately $65,817,772 - that's $9,092,525 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $8,838,787. 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 $52,259,591.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.