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.
Basquiat.liam, TryHardGamer115, Indigo, A Combine Elite, mrbradleyp, Tony Slark, england is my city, MisFits, LordChubo, HyperB, iamkaixa, TrojanModz, duduklusener, Urie, Marvin Megavolt, Marvin Megavolt, DigiGriffin, MordecaL70, tae52531, Newfolder, Jantam11, Creative oyuncu, clansky_net, lambara123, DarkStrike, Zethos, nagy.david995, Phantasma, Kuba6004(PL)(PL), Wolfish EL CUCARACHO, kaiocezar9, CheezMan, Tcisal, Prike, TabascoSauceBoss, CONRECON, Dune Jumper