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.
N.C._22, MTF Unit, lucasschumacher16, lucas31715, jaredtea2010, jonas.kirsch40, xX_Ewan_Prise_Xx, szaszimo76, szaszimo76, elperroxd *******, flo.rup, lillfear, lillfear, resadhesenov195, exoskeleton, randomperson, 去他妈的你自己, Dj Drone 0_0, sushiwaifu, hristya.vaksna, Minge, COSMA, n1k3~, vinauta13, dimitrijevuckovic026, coolhusky77, Shiruba, AYMAN_cdDesk, donric9109, gamingnicholast2, brittanybrill559, brittanybrill559, tornadoxddddddd0202020, vlk1989, ya zabil 4to eto ti, Łakzy ( ͡ಠ ʖ̯ ͡ಠ), piccolavale1989, lexa.sibaev, josh456, tim_10x, lexihad698, dramjattan4, sHx ファントム, StolbHunt3r, mauric.iofabianrivera8, Vanya_Fedor, Alcina, Vlados33YT, kmehha7467i, [TF] Miscamingone974 +354 others