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.
Arkane_Dragon, dbrett238, merkur0, Anica the bunny, Sam, geekdusud, buildermaster, Ninja Assassin, saschas1502, fps-creator-glo.ucoz.ru, Kinguin.7355608, GamerNuke954, Admiral Ozzy, josh_da_ninja, Rainbow Cat, [SGA-X] Bell Cranell | Faser, [XCO][CDG] Rainbow Dash, dalek3444444444, Tok'ra, Rodney Mckay, fldsmdfr4, fldsmdfr4, [F2P]loke, l0, 5Jango5, Arwwarr, halflifeunlocked/unlockthexbox, TESCOS MANAGER, Tryingdieing, Tryingdieing, Omega Taskforce, Ǥяα, RubberDuckiesOfDoom, Skeleton, sabresk258, тѕсм.[SG-1] Teal'c