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.
Momon-Wong, Everar, ATLas, Sacapetroleo, hasibkhan196, thienxa1999, falseprophet141, SixAxisNate, My Name's Not Alfie, SyncXavier, Sneaky Snake, UNKNOWNPENGUIN, starsforlife.rl, A e s t h e t i c, Alarm, BotW Link, Noobish Noob, StealthPaw , Skeleton, Master Turret, CheezMan, I'm an engineer