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.
j.paruch, JSK, StarJi, metrald1, danilstarbird, Mystery_Culprit, , sisa.d, alex2001sonic, goharian1379, Heckelex, jankowalski41, марио1108, марио1108, Shadow, GalvazimGX, Jérémy 69, Front.Face<AFP>ARMY.