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.
waffeln, Mateo ❤ ZakuGroup.net.pl, MattTheMatt, _PvP_Killer_, zakkay673, zakkay673, Febz, corecraftx, xKINGHARAMBEx, BananasTheKing - FROST Owner, Sk8erace1, TheEarthkin, Bonkersknave, Украинец v2, Nurds, cameron4x4, Vaerik, [OG] Fawcett, brewstermw, ProXx123, [FkG] Fischkopp, [GGC/MRC]Dieter, [CodeurGmod] ElGringo, Dennis | kickback.com, Rising Darkness, [SS-B]Ice, Kermit The Killer, TheElessaro, KrZyChU, Cat God, vashie, ipadlover8322, s0nichedghog, [Larrybow], DaViCyYo, PR0D1GY, thequadproject, JokerStoleMyBike, SnoopySleeper, Reuben, MC with PhantomX, cdog1019, SmashRaft, Cobbs, Dom Gambino, Codeah, Shynix, soccergabe77, WolfLP[GER], Cloud +85 others