mission_-_kill_breen_ii.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
          • Mission - Kill Breen II <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• Mission - Kill Breen II/
  • maps/
    • graphs/
      • zombiesurvival_b5.ain
    • soundcache/
      • zombiesurvival_b5.cache
    • zombiesurvival_b5.bsp
  • Read Me.txt
  • SAVE/
    • half-life-528.sav
    • half-life-528.tga
    • zombiesurvival_b5.hl1
    • zombiesurvival_b5.hl2
    • zombiesurvival_b5.hl3
  • zombiesurvival_b50000.jpg
  • zombiesurvival_b50001.jpg
  • zombiesurvival_b50002.jpg

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