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.
Manani, SkaaroX, masynykot, naufalafif2015, [RG] Outbreak, [RG] Outbreak, TheDinkster, starfy, ★MasterArtur★, blyapi3dec _3a4To, sers701, Logische Polizei, Lolče, Ellie, I Like Duxks, roman, MR_MURPHY8, lmaobox, GotSquils, Christmas Games_liker, Christmas Games_liker, aleksansander, Hunter, DasBoSchitt, xR3LENTLESS, brimiller74, xxjdxx, Cat, Cat, DarkSthi, InvisibleBran, Mr. Murphy, My Name's Not Alfie, Bruce Lee, Vodkaman, Riebulka, ♔FDGodZ♔, utkugulkaya®, Kilroy, martinxd7, Ku7E_Zombie, fantome, Eddie Dollins[Barnes], chiken the chicken destroyer, Kaniel Deem, [ER] VoRtExY :c, [ER] VoRtExY :c, [ER] VoRtExY :c, Frank, sethnace +53 others