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

• lens-flare/
  • info.txt
  • lua/
    • postprocess/
      • lensflare.lua
  • materials/
    • effects/
      • lensflare/
        • circle.vmt
        • circle.vtf
        • ring.vmt
        • ring.vtf
        • smoothring.vmt
        • smoothring.vtf

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