If Half-Life: Alyx generated the same gross revenue on the Epic Games Store, Valve would keep approximately $107,395,323 - that's $12,872,302 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $14,508,453. 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 $85,332,643.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.