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.
maks.toni, rsbrock20, gabrielucho1, marciohenriqueguedes, kon_talos, [A]rtifice[R], spaecii, Jeffrey, Mysiu Pysiu, (S.o.C)ShadowK3, DragonX1421, Arm'Zii, Thal Verscholen #TeamHeavy, -[SaDA]- NuLLCherry