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.
my_beer, rodneyneerings63, balogh.csaba2008, marshtoshow, gasztonyiviktor, Noobxd_5, Comet9985, master rex, aick1992, SLAV7EN, nirgabi, STUPYFY, david221220, BlackGhostTR, ishidon_hamham, mrbankchanel, Salivander, d24tic, ็็็็็็็, blaisedyc2, RodriTFFF, Pineapple, Candy, alandiegobr5, nesoyuncu, TheBeardedWonder, DegiNex, polux, iyegorych, Monster, gameingrussianalaska.com, AjHiN, THAI EMPIRE, ahlanda22, Link, Spike RO, casperbc123456789, dontrage77, kvnkssi, bynyjbybyn39, Chycol, fawwazpedro, Professor Dank, CodeMania, now you die!, djoens2000, cj02, Romestamo, shaun0838, giorno +1212 others