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.
Sightless, bobcataxe252310, qweewq3214, [RFAI] Strayed, cadu3195, Chalonga, TOO-BEE!!, TOO-BEE!!, TOO-BEE!!, herbtea1981, Sir_Heavyman | ,=,e, hewlet, joyrobotking, Kaboom36, TheLynxster, Kanade1337, ☣ AlexMFV ☣ CS.MONEY, Butterygames, O)_(O, juanelolo, [Mhkek]Simply Doc., DaRkWoRlD BTC4KEYS⸳COM, bb