This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons 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.
Ruben-san, TangoDown, Aderic, costicraft, NouZ-Bi, [FR] Tynaluto, MaxPlayer_, Daniel, Mikk, biolox, Meat, [HS] Craig, SmileGhost, Hades, Wido, F Ü H R E.R, MexicanRaindeer, Killer Memestar, -LT-4N0NYM0US, Tony_Bambino, The Proffessor, [RG - O] GordonJ/jdogh, Oswald, Grozger Zasrogtiveslin, Amélisse, #VAC_ConnectionRefusedDetail, Gandalf the Ale, Ash, SonOfZeus, Rockstar09, Megazord Prime, Optllizer, [RG] RoN_, [RG] RoN_, SonySeva, SonySeva, crazyscouter, Alex, Alex, [E-G] Exylar, Zino2201, lando calrissian, StickmanJohn, Newan, Furion ᕕ( ᐛ )ᕗ, Kaathe, [VDT] AlexDR15, ᐰSGC/atlantisᐰ, Bobbylol, BHG | Miscum Pascuerito +20 others