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.
HaloPotatoKM, cloudftw113, anguschan1016, rtgamer10 Chaos Master, Blues, tax evasion, Firestarter, KHò-ààAHH !!, Businessleader, roxmbmea, R4NG3R_ZA, robin.skold, internal bagel demons, Cherry Motherfacking Pie, Alaura, ryantmoore02, saphire2610, ◉XenoN◉™, KennethGraham19, Greenyred, gtamike_TSGK, Alexliam104, Critical Error, |Left 4 Breλd 2|, sinka1031, RouterWhisper, Fluffy innocent bunbun~