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.
danmar, Foffer, saevan07, ๖ۣۜBۣender, Wabba_jack, Friesberg, Zyd, |-ICE-|xDeath, YorKa, sam1995agent, TitanMnsM, Equester, [DN] nxtgenhunter, screwball, screwball, Belamir, davechanderpaul, [UG] Noffe, Wrench, Thedarkelf, Ness, Deltafx4, ProjectDiesel, Pipodebeuker, chancel23242, shanebob36, SrVendettta, >>[Astaroth]<<, The sentry guy, Nex, kknight2, jeremydailey55, [InvertGaming]Subflixx, [InvertGaming]Subflixx, chriselvisg, Chibi Chan, , Neo, SrgtPhil, Shady Figure, swaggcoen, Shy, KellanKelloggs, taylorroddin, Ace_snoopy, F0X40UND, xX_420_levi_420_Xx, Raylex, Mozilla Firefux, pr0phEt +10 others