Place the garrysmod10 folder into your Steam directory at C:\Program Files (x86)\Steam\steamapps\sourcemods .
Garry's Mod 10 , released on November 29, 2006, was the first commercial version of the game published on Steam. While the current version of Garry's Mod on Steam is the most up-to-date version (often referred to as version 13), you can still download and play the specific "Garry's Mod 10" experience if you own the base game. Official Purchase and Installation Garrys Mod 10 Download PC Game
If you specifically want the nostalgic "Garry's Mod 10" version, you must first own the retail game on Steam. Community members have archived these older versions for use as "Source mods". Place the garrysmod10 folder into your Steam directory
: Minimum 2 GHz processor, 4 GB RAM, and 5 GB of storage space (20GB+ recommended). Official Purchase and Installation If you specifically want
Restart Steam; the version should appear in your game library as a separate entry. Game Information : Sandbox. Developer : Facepunch Studios. Publisher : Valve.