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

• greport/
  • addon.txt
  • changelog.txt
  • lua/
    • autorun/
      • client/
        • tdlib.lua
      • greport_init.lua
    • greport/
      • config/
        • config.lua
        • lang.lua
      • gr_client.lua
      • gr_netshit.lua
      • gr_server.lua
      • vgui/
        • gr_derma.lua
        • gr_fonts.lua

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