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.
Rabos-San-MLG, Yuri, ariano113, fgt1290, Zen56, lolyo, manfromarz, CapiiWilson, artur.kononenko.04, edenarmar917, Joeythefox, Gamer Pro, ProjectPlague#DropTheBeat, ProjectPlague#DropTheBeat, Diggingwolf2605, Alex, bubi161204, Sonicteam998, FR13NDS| Feandel, Mike The Comrade