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
          • garrysmod <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• garrysmod/
  • garrysmod/
    • data/
      • adv_duplicator/
        • Mordonus/
          • space base by mordonus.txt

sergiomagana2013, , russ, FS Prone, dunno6376, alexsuf, kida0208, eldridgeedd, [TBF] Nightfire1989 [GER], yeffersonbarros27444, 14fighter, vannismith01, duckytheduck024, BE GONE OOF, slrfm, Ronnie Paints, Große Führer Tobias Sevcik, eliseev-valyusha, braun1070, hunterxy, Dark Telamon, jeremywn4, {MLG} xXMonsterHexXx, Nemesis711, EnderGuy, cKrisis, manoelmessias.j.santos, aa8326138, 【† -blackskull12321- †】, Espectro15, ppz, nolanmartindale, goran.rakovac, wohlddf, tariqsw69, dejvid.felenda, Assassin{G.E.W.P}, jm-bel, ♫FancyCat♫, radudcristian, zacop01, iliassiz99, Glouglouglou, adsdas.dasdasd.12, mbielema, [FR] Zane2504, rhyfan/cringe/dismyname/dafuq, kobolotin, TheHornyDragon, xxxbraesarxxx +40 others