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.
dominecraft.varga, Ez, mossesx, l2ksoah420, l2ksoah420, bubble, taner08051977, Gaming Freak, xx_novakid_xx, Spyro The Dragon, General Luck, jay.chandler.616, mzap560, longho773, ethanisaacs95, arkhamfighter, Ayumuluffy, xDarKStep, aleksei2003shpak, andresgaymer152, Topy, adam10robertson, Rick Grimes, ultraplush1987, NoNaMe69, NoNaMe69, SaVage_ShinObi, fightherot, dostpold, andresdeleonpaniagua1997, alzakdg, Dante Freeman 101, SixRAYGOD, christandoniego, byron_sepulveda.r, Gus Bot, www.jak1213146, Little Crumb, Leo, deadbyday, Squeeze, nantawtgolf2499, BMGaming, H T A E D lmao, artem.logumanov.02, pedroofera52, nagymarkl09, kyshon25, HamidikovAI, patatex111 +77 others