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.
Overused Bender, [TheArmiii]ItsArmiii[West-Side], [SGA-X] drachenball95, twitch.tv/sword1hammer, GrimeyBeets, Shady, Semen and onion sandwich, Printed Paper GλMING, M A L W A R Σ ツ, Chloe Price, HotDog, Tyrone Mufasa, Phys Frobro3.0!, Jakobeanz, Jeckyll, WhiteDeath, sgtblunderbuss, MrThankfulStingray, brutal rubber ducky, Tide™, Jerel, over00lord, Satoz, ラッドサミー (Radsammy), Ruby and Sapphire, true american patriot, |DG|SF|Skylordz11, i3G | Dev, Vesi, ≛Ły✯иж≛, Claude Auditore, RitualZebra351, NathanTaylor407, Fluke, p[r]otato, Tomekb530, Daemon Blackfyre, [XG] Devildog920, DerModMaster, Le Derp Rayquazus, OverKillHard, OverKillHard, MαŦᖇＩЖ The Unknown, Kallum, Jycaer, [ST] SchoolBoyAlex, [/CC/] BlazeHazzrd, Annoyance, hed şatçı mehmet, Den sorte Mamba +36 others