If A Story About My Uncle generated the same gross revenue on the Epic Games Store, Gone North Games would keep approximately $6,265,228 - that's $1,376,979 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $717,986. 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 $4,888,250.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.