Photon: Lighting Engine by Schmal

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

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