If Grand Theft Auto: Episodes from Liberty City generated the same gross revenue on the Epic Games Store, Rockstar North / Toronto would keep approximately $11,197,114 - that's $2,256,392 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,390,516. 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 $8,811,341.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.