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
          • Bbq Scorpion Sauce <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• Bbq Scorpion Sauce/
  • info.txt
  • materials/
    • models/
      • items/
        • BarbecueSauce_Diffuse.vmt
        • BarbecueSauce_Diffuse.vtf
        • BarbecueSauce_N.vtf
        • BarbecueSauce_Soec.vtf
  • models/
    • items/
      • barbecue_sauce.dx80.vtx
      • barbecue_sauce.dx90.vtx
      • barbecue_sauce.mdl
      • barbecue_sauce.phy
      • barbecue_sauce.sw.vtx
      • barbecue_sauce.vvd

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