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

• Displacement Cannon/
  • info.txt
  • lua/
    • weapons/
      • weapon_arc_displacer_new/
        • shared.lua
  • materials/
    • refract_ring.vmt
    • refract_ring.vtf
    • VGUI/
      • entities/
        • weapon_arc_displacer_new.vmt
        • weapon_arc_displacer_new.vtf
    • weapons/
      • DcSelectionIcon.vmt
      • DcSelectionIcon.vtf
      • DcSelectionIcon_.vtf
  • sound/
    • displacer_new/
      • portals/
        • wowwwwowww.wav
      • weapon/
        • activate.wav
        • charge.wav
        • deactivate.wav
        • dry.wav
        • fiah.wav
    • msg_to_noobs.wav

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