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.
MelonsRule ツ, Cap'tain, Anastasia, Piløts, jjpisit, RedEagle, driver from drive, [TPG] Senpai Jibz, [LLRP] Mitro [LockDown], [ARA] Carb0n, Nightstar, (TG) BunnyLuck, SLYHONCHO, Rene^^, Lykor0, TheGamerPlay, §3 [O&O] Thomas, TOXICSHOTZz, Adiliteriss, TomGeek, Hugh. G. Rection, _Moondust_, TennButay, Syndrozz, MeEeE, BallisticGaming, Gazabor - NineLivesRP.com, Shysley, TOMÁŠ ŘÍHA, Deis, Arnold0 (Arnold Lezéro), Coca Cola Bear, ShadowManX, ็็OliverA, ็็OliverA, T.w² Militant Mario, remainer, kash register, Exim, YETIKILLER3000, Mr. DeadNight, Haazey, Java the hutt, Percey, s3od9991, Joshhhh, Joshhhh, AGC Omega, Clumen, BeatifulSapphire +1 others