This download looks like a lua file. You need to place it in your lua folder inside the Garry's Mod 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.
kovpoll2009, sdjalali1974, rashed321alomyan, DJ-sona, DJ-sona, DJ-sona, gunner.clarke1968, cO. ThePhantom™, makc.dudar, turkufiliz, turkufiliz, andyguerracid, JastPlaying, bertisis, mattroberts2k13, daniel1005, johnny.rico