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.
GoodNight key-drop.pl, beratoyun, beratoyun, FaiZuRiK, SolbaniXd, Usoki, SmokeOfShot, [CKL]Günther Virus, Nalwak_Youtube, Maxim Kuzminich, [T-SIX] Котлета, Sprite, enmanish, krittamateza, Deus Sex, Mofi, wk22 gaming | Trade.tf, Skewshall, Thomas