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.
Belzemoon, Varix, andresdeleonpaniagua1997, Gromada, Xioq [MojoCity], iZeyNeX, SlownLS (> slownls.fr <), Nicok007, David Porto, aly.totodile33, Blak352YT, Misterwanwa, • NacroIsGod™ •, absoluT, LeyTecK