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.
ii_S1lver, tmb686mc, "DWayne", kat_the_cringe, andresdeleonpaniagua1997, TangoDown, Biofryd(rus), bossbreaker45, swagnemite, LaloLegoSparkle, absza01, caique.lanzini16, zennat.3195, Sir Cornwallis of Cobbsburg, MacMan, TheReptil32, Lee.Purdy3, turkufiliz, andreaceppi94, andreaceppi94, VigorousSnow, SM3<k, Trixie, Salted Peanut 5, Franck299, BobVeka, audrius10k, elfrikidecaballito, =PP= 419, DR.WCROW, peerapongchua