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.
Gorkepork, Dr. Freeman, mossesx, /K/noxxy, TrueToMyGrave, Zephex, aldwindaaca, aldwindaaca, Luigi, rcw4320, Anu, papa gabe, gamerguy8973, Woozle, Hinglewingle Throatjuggler, gutten, Petit Panda0007, Bob, Death from Above, giovymaster290, Burak, The_SAROKA (ง'̀-'́)ง, tj5935971, Corn Cake ©, #LaCa 1616 [HUN]#[email protected], Jelinator, kangur845, buddyboy08, bobcho03, noahbro24, Galacta-X, Saysay, rpmillermd, eric.kelson, rrhill, MisterAlpha, ryogablade, Spikrathe, gaming55.teo, kbubuka, re15revisited, 土豪.nb.sb, Fume, Vincentor, faget, marco-antov, 부추김치, Tom, mesikas007, [SOT]Mead +188 others