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.
Alienboy, Jonny140, Th3Game_BG, weswes26sarge, eren36baba, Wλrlocke, tylerdavis1210, Squakie, Kubík05, Thatsouris, 5432, devanandma, InfernalBadger, ⋙ΜΔǤØ€₣₣€ĆŦŞ⋘, ispeedzmc, A fuckin' Nark, adamhodr, Alex12273, jakewalker462, martihernandezmartinez, Pollux666, ItsTheNightBR, far cry gamer, AlliesRandomizedGuy aka Mr.G, pjeys95, sebypotcoava45, Klouder, sieusao, IamHE, Littul Batto, bOnK!, Tomek205, StepBack_LT, IDK (Back at CS), smlmsa123, Glant, metiocaggi, v0keZ, Drew SlxyZ, Vexento, DerpRollins, ilovepokemongo123rd, jarvis-jarvey, Minisher, Little Miss Mouse (月子), lukatheking32, alb.cip.alex2004, Zaeldan, freegfxformts, Fishy +52 others
