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

• VoiceRadio/
  • info.txt
  • lua/
    • autorun/
      • radio_init.lua
    • entities/
      • radio_microphone/
        • cl_init.lua
        • init.lua
        • shared.lua
      • radio_receiver/
        • cl_init.lua
        • init.lua
        • shared.lua
    • includes/
      • modules/
        • voicechannel.lua
  • materials/
    • models/
      • Mic/
        • mic.vmt
        • mic.vtf
  • models/
    • Mic.dx80.vtx
    • Mic.dx90.vtx
    • mic.mdl
    • Mic.phy
    • Mic.sw.vtx
    • mic.vvd
• LICENSE.txt
• README.txt

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