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.
Piterox, AnythingTailoredForAFiver, reborn the first, germanfogar12, Ajren, felipegamer5674, $*(Hitoschi)*$, lil jimmy THE FAT ASS BIG SMOK, emir tezer, makakukata2018, deividasqwer, Noventa y Nueve, mic.dey, Valnixium, Sailenc3, dragancaki, IvanVladimir04, kajuknpplo, Turkey, Lazyengineer, MarianScheul, jorgearturo10dc, lolf5669, V92