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.
rossmcnl, Moretti Melo, Sheriff Rosco P. Coltrane, BigWangGuy247, Ken_Senpai, You're Aren't, You're Aren't, You're Aren't, mattpullen, mattpullen, zestanudru, BGZ7002, ulana kocica 13, thewitchofgeek, Gucci Pucci, vgr3alt, RayfromCT, maks.vlasov.16, edgarsgrivins, the party boy, Shybe, Falxo, aestheticjmack, grug, Red Baron, TheUltimatrixUser, acehighent, emoloney1, lilou905, Ruric™, rmep1978, IndifferentDoom, <O_O> DJ Z, eimaiskylos, burakdurna2733, kevinonledge, arcasso13, M A R C O.png, Chuck P., The interviewer, professor.bdac, sillonmarron97, Nugbot, jamesgray8, ＣＡＲＤＩＧＡＮ ☮, giustomax, LittleRedGriefer, Techiesplash, erick_chavez94, John Cletus +331 others