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.
Tulip, ezrealelloco, emirhanurer, tssthesplitscreen, romb.11122, al.azizs, noob0160, josemaria_zuba00, Tyrial, Hero989, pasionforglory, lachyt, ѦÐἴS, mackkibler, ivan.vrbic646, nishixot, alan.siglo10000, OrdoRavens, Perforator, Aer0EX, sebastianmorello512, @Ka3rA_freeman, pupper prince, 4terrancej15b, Mr.Pickles, lordravenx, Hellboy1216, Big Daddy, Thiago Gamestar, Thiago Gamestar, The Electric Cultist, turbbi264 :D, Braian_99, kristaps6484, Raccoonofstealth, Sexy Spy trade.tf, Mr blue, xx31315, The JMP, Ricon, LembuGhini, Braian_99, Squeeze, chinawut980, emozero, luisdiego_libra16, joncyblack, 15695956088, 0660198032, LOW +191 others