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.
Manio, [L²:RP] Vimpto, Ledo24, gigamgbo, WeHazToast, mattpeckron98, AurA, Kayxy, Kayxy, samuelnijman, ..., Sorch, rolandinhocraft :D, mattpeckron, ExcelFX, LIL TEETH, hsgr, Melon Le Prince, Oberst.Killerbine [9te PD], Lisztpofa, Tank, Itzutak, Pongo, Stim, Hobo Horse, [FT] PartyMaker, [SCG] Rainbow Anarchist :^), Lt.Cutecumber, jeremi, Murv, ̣^ίήνάĻίđ, Zenolisk, Yoh, MixMash, |RR| Supreme, ๖ۜ§tealthPaw , [SRP] Thomas, [SRP] Thomas, Jack, ζ MUCKER MλYHλM ζ, Shadow OverLord®, Shadow OverLord®, Andromeda, TEOTKU