(specifically version 9.0.4) is the final free version of the famous sandbox modification before it became a paid standalone game on Steam. While the retail version updated significantly in 2010 (GMod 12), version 9 remains a popular "legacy" version for those wanting to experience the game as it was in late 2005. How to Download and Play Garry’s Mod 9
Because GMod 9 is no longer officially supported, getting it to run on modern systems requires specific steps and "Source SDK Base 2006". Guide :: How to get Gmod 9 works? - Steam Community Download Garry's MOD 9 PC Game 2010