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.
dunno6376, metry.lok, gabriel101000, MaxPro2100, Miya-Chan, danilstarbird, SHMMITY, Knee, Rahpants, sasha.ermakov2003, SamyTheOrange, [LFG]ThePianoTheorist, War Chief, BeastyRhys15, Huzzah! Sir Huzzah!, jgvg24, killerv1210, shad3004, DENO, Quirox7008