If Garry's Mod generated the same gross revenue on the Epic Games Store, Facepunch Studios would keep approximately $158,099,926 - that's $17,481,811 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $21,422,717. 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 $125,665,851.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.