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/
    • military models/
      • apc car.txt
      • apc.txt
      • big truck milatary.txt
      • humvee.txt
      • little truck.txt
      • tank.txt

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