This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:

• steamapps
  • common
    • garrysmod
      • garrysmod
        • addons
          • data <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• data/
  • adv_duplicator/
    • !slot machine by drunkie.txt
    • gmflatgrass0044.jpg

ChickenDestroyer21, jonesboy1999, PoloMaster, FrankGammer, gassed up, jquub4343, ТРИГОН, OSSI.DK, minefalling, LaurBerg, xder, CRAZY_TACOz, victormiles31, LIL Sande, Tofu Service ★, fredericwow2444, Angelic Ross, Munshy, dima5241, zehra_klkci11, ᴅɪɢ (ᴛʀᴀᴅɪɴɢ), eriikesgameryt, Skylookout1, scamalitious, Читерила, [133O.T.B]Nololoqp, tomayerattika2, scouteedog, mikkel1234dk | Pvpro.com, BOT Fred, MinePixl, rbhbkkn.kmrjd, gat gat, Medic Frostin, [8CO] Orion Spartan, I chomped ur mum buster, I chomped ur mum buster, Alex,MaSTeR, Comet, Alkash / DarkSw2rd, Thomas the kurwa engine, pick.papa, cstricata, sabinarohani66, kevinloucks1310, Σ Ｃ Ｈ ♢, [DN] monstercom1, NoNeym, moonglow1970, ВОРОБУШЕК +1711 others