If Marvel's Guardians of the Galaxy generated the same gross revenue on the Epic Games Store, Eidos-Montréal would keep approximately $45,287,876 - that's $7,226,171 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $6,039,256. 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 $35,928,992.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.