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.
xdarksiderx1, sergiomagana2013, €KİNGSMAN007, fluttershyisbestpony, Ziddan, FormalSquid, FormalSquid, Night Hero22, gokupowergtyt, Satael, faizamfomegnam, UncleJimmema, falseprophet141, Wolf, DECOY panda1, DECOY panda1, Anubis, DrakanJR, Texugo Sensual, Булочка с хаслом, Poner, Vrbey, Emotional Glowstick, JeffreyWoods28, JeffreyWoods28, Strider, Danny, UncleSHREDDER, Alexander22512, Vinrax, SharkFin, leocardu, Warpath-the-drone swe, Warpath-the-drone swe, Hirasmous, jwnovosel, Jackie Saint, Wolfman19682005, Wolfman19682005, DukeClowning, SketchyBehavior, A Deep Commitment To Konata, Galle, TomeKK 2Shae[HUN], Baty