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.
AndyTM6676, Zetron79, AkainuTAMAKI | Proxy, AlphaQuest458sn, Grozger Zasrogtiveslin, Str0m, BaconPlayz, hollyhock073, DeadPool, cheuble, sentry_down, HyperHero6889, Micchi Derp, An Egg, GAMMA_ALPHA0, evoiwviowevnp, Wolfie9, Wolfie9, SpodermanPls69, andyzeke, Roman, berdan0, romanek.prihoda, SkyGuarders MC, ShinusThePanther, [LGMOD] RedBlaw / Luca, [SAO] Kazuto™ FR | gamekit.com, CONRECON