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.
amirgordon37isback, dark_and000, amirgordon37, 1qwferma, Lewis, Lewis, Lewis, XXX, GoodPeanutandGabe, charkenco.x81, VeryLegitName, 「【ληzø】」, Tails19935, Rooie Ronnie, Snover, QUAKEMAN00, dantepellico, kearley2, joegeran89, meliodasgamer6, RCT/DanK, Samy aissa el bey, Samy aissa el bey, [DATA THE EXPUNGED], Senpai, UncleSHREDDER, Spoobs Gohstly, Thestickystickman, Thestickystickman, Thestickystickman, WareWolffTen31, Dookatroopa