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, ▲ FaZe 2.0 ▲ Rainbow, Nraaax, Herr Spaztrout, D.ツ, mubasher.shaheen, Dr.IrishDan, Darth Vader, ☃❆[SM]RazorNight❆☃, SFAriel, Woky, Clockwork, Neon Umbreon, BOB DYLAN, Zuruuaku, DieAnotherDay aka Mr. Coala aka, James™, Rodinavian, Stealthy Zoroark™, PonyEB, Manni das Mammut, Joy Ferret, Solar, Yoh Sambre #Ero*****, Nidolai, Nidolai, , Mr. Moostache, [NL]Hero, Mr. Brightside, Kolorek