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
          • RetroText's Close Quarters Lighting <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• RetroText's Close Quarters Lighting/
  • settings/
    • presets/
      • colormod/
        • battlefield 3 close quarters.txt
• INFO.txt

ZeroKite, ZeroKite, ZeroKite, TipsyChica 3.0 #VLADIMIR, kalachim555, Қatka ♥, ｘｕｌｆ