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.
DoggoficX90 - DAT BOI, re15revisited, re15revisited, mattpeckron98, Koshka, Freddy, Flosius, Flosius, JAN.HYMR, Shepard, [UC]Jose76Tron, JCelmaestrodelpsp, SNW, adazie3500, double rekt, Альберт, Shaxer, Arthurdead, ^0Metro Cop, Dr._Hax, Col. Mustard, Col. Mustard, QuieT, Jinxed34, Sonic Alchemy, White Alien, Vergil DmC, Agentlulz