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.
A-l, ☢SCP-049☢, Wee Wee, pilot778, Foxy, F1SHfry, X_SIMOU_X, Ruko, Figaro Beatbox, ЕГОРЫЧ, cjw3751, cjw3751, XxPiotrek577xX, Mister Dogeowsky, Corbin, Duke, BAIDU™, tim.wolf.kond, Mr.Fish, Thomas Juan Rogohoon, Certified gamer soldier, vpantaleon659, Đémǿή, Đémǿή, ❄cardigan❄, GH05TL4ND3R, RaidenClosth, The Mighty Edwini, [NRK] PresCmdr. Sergei, Ditters, ASBUG, thestardestoryerandempire, ArmySoldier, Cyxapuk 228, 스팡마더, Spanish Dolfy, Kyuu, Time bomb, stas.lyubich, clay9899, tureogonzalo15, Snockza, Snockza, pipa, asiliria, Lora hart, Lora hart, HunteR, HunteR, 미군고양이 +323 others