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.
gibus, Bobcat, ุุุุุุFeWeR ツ(CZ), Chuck Maurice, GoogleChrome.exe, [LFG]ConManRoadz, •_Warsaw_•, Myles, jnorm, [TKZ] ANine, Handsome Turtle, Haven / ゲーマーガール, piro, ™PUDGE*JeBar™, Yatangaki, Haruka, Novalis, Eternal, [OM] Ninjaxy, kennyASS, Renco, Tilde, Селёдка, roach