headcrab_canisters_tool.zip

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

• headcrab_canisters_tool/
  • info.txt
  • lua/
    • weapons/
      • gmod_tool/
        • stools/
          • headcrab_canisters.lua
  • materials/
    • VGUI/
      • headcrabs/
        • headcrab_fast.vmt
        • headcrab_fast.vtf
        • headcrab_normal.vmt
        • headcrab_normal.vtf
        • headcrab_poison.vmt
        • headcrab_poison.vtf
  • settings/
    • controls/
      • headcrab_canisters.txt

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