If FTL: Faster Than Light generated the same gross revenue on the Epic Games Store, Subset Games would keep approximately $16,745,039 - that's $3,075,972 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,147,051. 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 $13,224,463.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.