This download looks like a lua file. You need to place it in your lua folder inside the Garry's Mod 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.
dmilejko72, j.e.ricagno.oliveri, zbigu55, treyton, dittrichveit, yebraa966, BlackMantis, Alfe [Fr], Ahoudori Kazuo, leonardbob49, Teddy_PL, masterlagartox, nowis_dark, Clockwork, Sneaky Snake, mattpeckron98, Erfan_NY, ssh99o927, Dax Winter, janusseanbgarzon, Mario44422 ヽ(⩾Ѡ⩽)ﾉ, Lee.Purdy3, FluttersIstDerBeste, maykol522, hamid4356, xX_MadPooch_Xx, some unfunny skeleton, dragos170302, TF2MlgEngineer #TeamHeavy, CONRECON