Garry Newman confirmed that Valve has agreed to let people export game modes from S&Box (spiritual successor to Garry's Mod) as separate games and release them on Steam.
They are also planning to allow people to monetize their game modes within S&Box, unlike G-Mod.
Valve now has a store page up for s&box (Think Garry’s mod 2.0). Gmod is what got me hooked on pc gaming to begin with and I can’t wait for this to release. https://t.co/FgwhNIbRqb