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.
clayrin456, Wolfishspy, alexissoldado, gabo.01xbox, pancholofrancisco6, Donopussy, kshomatic, Jerry, Staro, Entony1234, xBlueIcePandax, bellito-bryan, bellito-bryan, adan2017, daduka66[POWAAAA], cgrassiano, cgrassiano, Lineshothellcase.com, kmn8687, sprite drive, rosantosguelfi, Krimena, soldandandemi, pyroCanine, diehardtryhard, JuanValencia, vladimircabahug18, jesusdavid_150904, DoFrane, Sir hatty hattington, little wolf5, tssthesplitscreen, MrBeefy, hcolby0314, hcolby0314, Leo.strauss, ramirezabraham318, leosuper, kennystokes03, trocoli-01, KawaiiLM, camilomira2112, christandoniego, irivera2424, jonathandavila806, jonathandavila806, jonathandavila806, Carl Jason, doomatic17, JonathanB +80 others