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
          • Realistic HL2 Weapons <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• Realistic HL2 Weapons/
  • info.txt
  • lua/
    • weapons/
      • weapon_realistic357/
        • shared.lua
      • weapon_realisticpistol/
        • shared.lua
      • weapon_realisticshotgun/
        • shared.lua
      • weapon_realisticsmg1/
        • shared.lua
  • materials/
    • VGUI/
      • entities/
        • weapon_realistic357.vmt
        • weapon_realistic357.vtf
        • weapon_realisticpistol.vmt
        • weapon_realisticpistol.vtf
        • weapon_realisticshotgun.vmt
        • weapon_realisticshotgun.vtf
        • weapon_realisticsmg1.vmt
        • weapon_realisticsmg1.vtf
• Read me!.txt

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