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.
The Vik, cmchapin, [red]}BKP}[green]Kirill266, Asckar, Nostalgia Clovis, ❤ Amata ❤, Grozger Zasrogtiveslin, LoS TheGoliath, Tactic_N7 the Comrade, Moo-Roy, Skyfeels.ₑₓₑ, Steel, Siber, Hello Kitty, GamersCentury~DarkOfGeek, ajking01, MCraftFR, Stormy178, SC3 | Zeon, Tryston, ColdPhantom, Veloifaptor, [EGN] Solar_FlareHD, (SG)Nomad Roman, [53rus]*-*VegaZ, [=Elite=] Lukas ; Arts & Design, | AG | JohannesNeo, .:EvO Scorp:. P'TIT PAPA, ❄ ᴡɪɴᴛᴇʀ ❄, PotatoTurtle