If Celeste generated the same gross revenue on the Epic Games Store, Maddy Makes Games Inc. would keep approximately $33,944,616 - that's $5,616,818 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $4,492,448. 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 $26,905,944.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.