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

• Hornet/
  • Materials/
    • Models/
      • Corra_Ashu/
        • Vehicles/
          • hornet_bump.vtf
          • hornet_decals.vmt
          • hornet_decals.vtf
          • hornet_diffuse.vmt
          • hornet_diffuse.vtf
  • Models/
    • Corra_Ashu/
      • Halo_3/
        • Vehicles/
          • Hornet_Open.dx80.vtx
          • Hornet_Open.dx90.vtx
          • hornet_open.mdl
          • Hornet_Open.phy
          • Hornet_Open.sw.vtx
          • hornet_open.vvd

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