Garry’s Mod has #sold more over 6 million copies, #Facepunch Studios founder #GarryNewman has said.
https://twitter.com/garrynewman/status/510070397857824768
The PC version of Garry’s Mod was released much earlier than the Linux (just 10 months ago) and Mac versions so it stands to reason, and just by looking at the numbers obviously, the majority of sales came from PC.