katana

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

• katana/
  • info.txt
  • lua/
    • weapons/
      • weapon_katana/
        • shared.lua
  • materials/
    • models/
      • weapons/
        • v_katana/
          • katana.vmt
          • katana.vtf
          • katana_normal.vtf
  • models/
    • weapons/
      • v_katana.dx80.vtx
      • v_katana.dx90.vtx
      • v_katana.mdl
      • v_katana.sw.vtx
      • v_katana.vvd
      • W_katana.dx80.vtx
      • W_katana.dx90.vtx
      • w_katana.mdl
      • W_katana.phy
      • W_katana.sw.vtx
      • w_katana.vvd

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