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.
dljackson2987, [TeamTurbo]vTeq, mertmelih, Petrenuk, Masonclark041907, di-ra2016, davkl17, lycith623, ivan_power88, Blue Inkling, pablito, slayerfor, Gekk0, liberplays *Z1K4*, Wido, phillipmolina142, fightingcoward, A Tactical Error, adnanchicha1983, alvarzdaniel, Dosh, 夜ト, Lince Bélico, Doge, #VAC_ConnectionRefusedDetail, Rainbine, MrFrig u.u, Googly Goobly Cat, WolfKnight, Playr19, DeadlyChunk, ive been lookin ut it wit ma hed, Wreckinges, GTDriver1230, supershadowglitchy, Niggarto el Negro, Russian Queen, [Fondateur ElysionRP] MrDiaboloz, Quindlyn, LogicalBrick, Lørd Tyler |-/, kaiseraso, Space Cow, Wet Shoelaze, Xaar'lien, cdaicuong2210