If Black Mesa generated the same gross revenue on the Epic Games Store, Crowbar Collective would keep approximately $35,933,392 - that's $5,910,615 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $4,763,644. 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 $28,487,925.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.