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.
TAILS988, Tubbz, [T.A.B] Springtrap, Nraaax, Herr Spaztrout, D.ツ, Dr.IrishDan, Darth Vader, ☃❆[SM]RazorNight❆☃, SFAriel, Woky, Clockwork, Neon Umbreon, BOB DYLAN, DieAnotherDay aka Mr. Coala aka, James™, Rodiogor, Stealthy Zoroark™, Manni das Mammut, Joy Ferret, Solar, Yoh Sambre [Le CaCa !], Nidolai, Nidolai, Feverish Reverie, Mr. Moostache, [NL]Hero, Mr. Brightside, Tetrox