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.
SilveroniX, |WZ| H3X G4MING (hellcase.com), LETHAL CHICKEN, sarasincat666, erayesendal02, gaelgerardoramosdelgado, Shadow l KryptonNetworks, shawN, veltrollex, Cygun, Kingsmantwiin, Ilya, JohnGon, ♚ Peachy ♚, kacperpizza, A E S T H E T I C, mmm I liek dat, Atomic, TEA Chili, Monkey Lover, jotpegiePL, Alex, Victorian Seagull, Electro_Speed, Ratchet, simon11, simon11, plastovnik, As a kid I looked like a carrot, myriad1973