This download looks like a map. You need to place it in your maps folder inside the Garry's Mod folder.
You should end up with a directory structure similar to below
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
the classic, bagastribismasaleh, re15revisited, re15revisited, ariano113, Ivλn, Ivλn, khushsikka7, Kennedy., raymond1922, Gosman, skubivic03, Dead Meat, judge judy, |Konamichs|, Keithy, Z4P, Sabun, Vincent VanGoat, HALFLIF3THR33, RyanTheNerd, 95Navigator, ★OMG Star Lord Iba Deepsea, dobro, dubstep, TheQuackSavior, DarkShadow126, PhanPhan морской слон, HSn, [BB2] Faith, AWACS Space Eeyore, bunny, Johnny Five Aces, Judge Fredd, Xoma-X4, Xoma-X4, UltramanUltimo, raygun gothic, [TSC] Father Smashmas, Soviet Comrade