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.
>JMUZZ<, Eterna, gomynkyl123s, ch0w, DilutedSalt, Valenchenko Kieve, Sarge, morganbrown2, Turtle, NEGRODAMUS, FoxDark, Agent, Vvapid, Thorn, Username, Salty-D, Jim Lahey, Milltaa, mattykat12, That Misunderstood Canadian, ISyncDR, gheorghe.cristi, Cranberry Jam, FireBlackcat, Tyler, ☆-Goofy-☆, [B2G] J:an, Smixed, Niclauke, ahmed001188, WifebeatingTowelhead, 猫精灵, Pink Guy, [TAU] | Cheeki Breeki, JpZ, CyberGhost, Rire, Vtgordon, Nikko, DENUGAN, Mr #Nye ip, Anon Hova, Skynix, PhanToM AnGeL™, Leyf, Mr.FoX, Nimimerkki, bowenggarthlyyy, nmi.020604, themattyboy64 +115 others