If Baldur's Gate 3 generated the same gross revenue on the Epic Games Store, Larian Studios would keep approximately $645,779,642 - that's $61,816,331 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $87,924,497. 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 $513,592,897.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.