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.
Bylixglave, Sensation | Trading for Keys, Barkolas_TR, PezzaFleche, romain.vecuaud, colepagliuca, Bleedix, Nicodo123, Prox_HD.FR, colserra.gaming, Swaziland, DeathMachine128, | Yusuf Bey |, Killian Alanzo, -|Athreal|-Recreating LethalRP, MecroIntention, D7OM-VIP ♔, Benlau | Koxon, Chris CSGOEgg.com, The Freeman, TheMakkSniffer, Qc_Fr_Gab_le_boss