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.
jrjes, Nekyto, Nekyto, [SP] Sorest, Bad Morty, temokotorashvili, Rayanezequiel9, Captain Charger, karyloove, od.danil2000, diaclone09, A4MS, Thirst Silver, TheSpaceVirus, David the Coffee Craver, SirTalis, SuperDude34590e43q567423774974, George08, ⊰UTNerd24 ❤