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
          • L4D2 Evil Dead chainsaw hexed <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• L4D2 Evil Dead chainsaw hexed/
  • materials/
    • models/
      • Zeaposs/
        • v_chainsaw/
          • chainsaw.vmt
          • chainsaw_chain.vmt
          • chainsaw_exp.vtf
          • dhainsaw.vtf
          • dhainsaw_chain.vtf
          • dhainsaw_map.vtf
  • Models/
    • Zeaposs/
      • melee/
        • w_chainsaw.dx80.vtx
        • w_chainsaw.dx90.vtx
        • w_chainsaw.mdl
        • w_chainsaw.phy
        • w_chainsaw.sw.vtx
        • w_chainsaw.vtx
        • w_chainsaw.vvd
  • readme.txt

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