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

• gdr/
  • gamemode/
    • admin/
      • admin.lua
      • cl_admin.lua
    • cl_init.lua
    • cl_scoreboard.lua
    • cl_voice.lua
    • init.lua
    • rtv/
      • cl_rtv.lua
      • sv_rtv.lua
    • shared.lua
    • sv_afk.lua
    • sv_messaging.lua
    • sv_rounds.lua
    • sv_spectating.lua
  • gdr.txt
  • icon24.png
  • logo.png

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