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

pc.game246, Lowkye, RedShadow3060, sergiomagana2013, hasanbanz3, Mr2468601, pukeyworm, Full_Stupid, Mr. DusMees, Mr. DusMees, dragancaki, jtrent238, Dalek, lukaszmoranski150, mossesx, |ZENONIK|, Nero7634, ! Scotty | Aiden, .:HamburguesaArcoirisGalactica:., Danny9k, [ϟϟ] TLG-EGL | CONFIG [ϟϟ], JakeFromStateFarm, Deadly, Jaszczur Z Kosmosu @Liveserver, marvinrenner, Sennatore, minecar, ThaaAlex, Grosteni, athariqbal, TWICE, bynoment, Connor | D3S DESIGN, Mew , >Razor<, Xcitie, Bumbastic, Frank Underwood, 789tomi789™, Hemmuli1, arek.zydek, [U-GR]Pinka, lohanmota01, Dante Freeman 101, þřomαřş, min020522, Seven9, Recrouit, Julian Musk [gUn], wakil-2006 +60 others