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.
Toxzic, danial3141, Toaster Oven, ViiPeR64, [DCE]Ipay CSGO500, Legion Riven | RIPPER, \/endetta, Lord Potato, pvt Johnston, pvt Johnston, santi.damico, 한국여우, 한국여우, money93577, nafikov.vasil, PedoRiegel, kiralyszabolcs10, Orion, Woozie, Enux [DC], Demon, Demon, theglobex17000, MiniPilot, MiniPilot, npragland, =LXN=Snowlep337, chugu, Reverend Reaper Falcon, ᅚ, mate19, LoopY, NATION X, albertofuentes2002, Xillenn, |-KPH-|, QuasarBarkas, QuasarBarkas, QuasarBarkas, funkyblackcat17, Spess Mehreens, plastovnik, JetraHD™, kyle.stokes321, _X3n0_, Macwarswas, Macwarswas, Селёдка