If Half-Life 2 generated the same gross revenue on the Epic Games Store, Valve would keep approximately $35,136,585 - that's $5,792,905 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $4,654,989. 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 $27,854,102.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.