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.
Justtom, Ne'Ko | ✪ - mirakapz97 - ツ, Theta, Maud Pie|P_INC, Pandiox, EgN| Danker Pepers, [IJWTB] Thomas, Denis (Governo Nerd), Jrock, emmaaa, skittleswow9021, PicHZaz, Slodrein, I Я ДШЁSФMЭ, Ruby and Sapphire, [IJWTB] Noorquacker, ۩∆ŘŤŰŖ ĶΩŔŇỬΖz۩, Pewkek, |DG| SaintlyDemonic, Kanersps, [VXGGO]VORTEX, CCC (Owner)Crazy Chris, Traine01, Fish, GalaxyHighMarshal, MR.EPIC, whatsup1827, Tomas, [FR] Matspyder, DefaltNF, Folgers Coffee ©, ζ MUCKER MλYHλM ζ, Gamergasm, derrysneddon02, ❄ ᴡɪɴᴛᴇʀ ❄, der zylinder, Larry the Athletic Apatosaurus, I'm an engineer, legal meme guy