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
          • gmodjp_0.9 <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• gmodjp_0.9/
  • resource/
    • fonts/
      • akbar.ttf
      • akbar.txt
      • coolvetica.ttf
      • coolvetica.txt
      • csd.ttf
      • csd.txt
      • readme.txt
      • TF2.ttf
      • TF2Build.ttf
      • TF2Professor.ttf
      • TF2secondary.ttf
    • game.ico
    • gameui_japanese.txt
    • garrysmod_japanese.txt
    • HL2MP.ttf
    • language/
      • constraints_japanese.txt
      • construction_japanese.txt
      • faceposer_japanese.txt
      • main_japanese.txt
      • materials_japanese.txt
      • misc_japanese.txt
      • models_japanese.txt
      • npc_ents_english.txt
      • poser_japanese.txt
      • postprocessing_japanese.txt
      • render_japanese.txt
      • sandbox_cleanup_japanese.txt
      • sandbox_hints_japanese.txt
      • sandbox_settings_japanese.txt
      • sandbox_undo_japanese.txt
      • server_setup_japanese.txt
      • settings_japanese.txt
      • wheels_japanese.txt
  • âèü[âhâ~ü[.txt

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