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.
xycvdf, schkrisz130, molhantbeeckelaers, abdulheyvahap, Crazioz [Co-fonda RoyalRoleplay], warrengaming, Arbofurmich, Rocket.Woods, g.l.n2001, GяooیTeaM ☆ Ŧallio Ushikawa, QTG坑, MaverickRat, kangyeran, Master140, Trager, ddhs5845, NS1205, panther73m, Naga, Pink Guy, astabrother