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.
graslukasz, JoshShadow123, Ahh_KimoChii, eliseev-valyusha, maxbruno10, bakaWOOT;w;Sugoi Desu Ne, shellcracker, LazyLama101, Sassy the Sasquatch, DR.BEN, jose93, SinisterSpud, ali716p91, ilyasrecovery32, busermanov5, garlind19, vanek.nikishev, ⎛⎝⊳ⒸⒶⓁⒺⒷ⊲⎠⎞, mikecoros05, Chibi Chan, glimmerman, BoD VenomZ, Hirasmous, =PP= 419, Ignas, wheresmygui, [BA] Brucektrain, WHO ME?, Datthing101, chen fixit, Sir Oswald Mosley, Dune Jumper