If Life is Strange: True Colors generated the same gross revenue on the Epic Games Store, Deck Nine would keep approximately $16,785,287 - that's $3,081,917 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,152,539. 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 $13,256,478.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.