The developer has confirmed that he is working of the Linux version
Garry's Mod is one of the most awaited applications on Steam for Linux, and its developer has just confirmed, sort of, that the game will be arriving on the open source platform.In case you didn’t know, Garry's Mod is a physics sandbox based upon the Source engine used in Half-Life 2. It's only natural that a release of this mod will follow closely on the heels of the single-player game.
A player has received the visit of Garry (hence the name of the mod, GMod) on his server. He asked what he was doing and Garry said that it was testing some Linux features. The detailed conversation can be found on reddit.
Garry has played a very cruel prank of the Linux fans, on April 1, when he announced that the Half-Life 2 mod would be arriving on Steam for Linux soon.
Even if he confessed for the joke, it seems that the game is not that far off.