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.
shizozavr, koroglualperen014, florobely, furiya6918, serkan_guler18, asmura, risoskol, Manual_Control, aronlollollol, ziecenaka, colbypopkie, galusins56, Sindroom, kdragon410, |eG| Hyperion-Tiger, chlrud951, Ridiculously Scary Skeletons, GetsugaGamer, Mi40, Junior_Gajko, hel2mes01