TF2 Rifleman's Deerstalker

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
          • TF2 Rifleman's Deerstalker hexed <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• TF2 Rifleman's Deerstalker hexed/
  • materials/
    • models/
      • player/
        • items/
          • sniper/
            • deerstalker.vmt
            • deerstalker.vtf
  • models/
    • Deryer/
      • items/
        • sniper/
          • sniper_domination.dx80.vtx
          • sniper_domination.dx90.vtx
          • sniper_domination.mdl
          • sniper_domination.phy
          • sniper_domination.sw.vtx
          • sniper_domination.vvd
          • sniper_domination.xbox.vtx

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