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

• DerpRP Inv/
  • info.txt
  • lua/
    • autorun/
      • client/
        • cl_drpinv.lua
      • colchat.lua
      • drpshared.lua
      • server/
        • drpinv.lua

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