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.
alexisdovlatis, Scales, KONO DIO DA!!! ゴゴゴゴ, 9_isabeau_6, Twy (no sound), Twy (no sound), amangtua, unoriginality14, rocky balbulbasaur, レム (Rem), Cxero, MrKwasny1519, MrKwasny1519, A Cranky Lady, Notorious Small, Skyline, Freddy Krueger, Dr.Demo, joeylife1, Beep Beep Evolution Party, Explosive Eggshells, Dj.Spread'Em, ☢STALKEROS☢, ZiPER, NYan, Yunocchi Rifleworks, IcEmAn911, maximusxj, Ein, Max.exe, Max.exe, Dark Emperor Obama, Hirohito, Killseeker, narya1, narya1, ❤DJ Sona❤, ❤DJ Sona❤, ❤DJ Sona❤, Glennicus Magicallicus, marigi64, RavenSRC, neko, neko, ayylien, CrazyBooze, Dorian[FR], Xoma-X4, critson, critson +3 others