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

• Acid grenade/
  • Acid grenade/
    • info.txt
    • lua/
      • entities/
        • AcidGrenade/
          • cl_init.lua
          • init.lua
          • shared.lua
      • weapons/
        • weapon_acidgrenade/
          • shared.lua
    • materials/
      • VGUI/
        • entities/
          • weapon_acidgrenade.vmt
          • weapon_acidgrenade.vtf

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