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.
D4NIIZZ, gamehag.com, $ElNAtOpA$, adamadamfraschetti, cristiangiraldocano22, kimsors388, nicageniusly, matiascofre619, benigabbo, UnoPluto, princemurf77, Donutzillaaa, Anonimo539, jhoanandrehilario, Nikita, yamilbordon56, Squirrel Cereal, austin-handrahan, _simon_sanchez07, totoayseali, FelixMan_470, FelixMan_470, mostafadarvari62, hamzaaaras77, hamzaaaras77, hamzaaaras77, chipiri295reyes, Stodgier!, moogelba7er $csgocases_com$, marcelo.espert, aironpajuera, metrocombine2231, meloptido, laguafrancisdominic, thisismyname, martinezromeroleon, BODAT GAMING, stevenvrley8, elrodrigo2009jjd, Mr.F1zz, pringoglenn, neiltamboy, homero river, alexbob10010, DRAGON DEEZ NUTz, JustaguythatlikesSCPlol, uadachinuisasha, jonasvein, Arkxn, monir +981 others