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

• Rickroll Grenade/
  • info.txt
  • lua/
    • entities/
      • ent_rickroll/
        • init.lua
        • shared.lua
    • weapons/
      • weapon_rickroll/
        • shared.lua
  • materials/
    • VGUI/
      • entities/
        • weapon_rickroll.vmt
        • weapon_rickroll.vtf
  • sound/
    • weapons/
      • rickroll.mp3

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