If Rebel Galaxy generated the same gross revenue on the Epic Games Store, Double Damage Games would keep approximately $2,571,272 - that's $621,396 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $214,264. 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 $1,949,875.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.