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

• Portal_Gun/
  • info.txt
  • materials/
    • models/
      • weapons/
        • v_models/
          • v_portalgun/
            • v_hands.vmt
            • v_hands.vtf
            • v_portalgun.vmt
            • v_portalgun.vtf
            • v_portalgun2.vmt
            • v_portalgun2.vtf
            • v_portalgun_exponent.vtf
            • v_portalgun_glass.vmt
            • v_portalgun_glass.vtf
            • v_portalgun_lightwarp.vtf
            • v_portalgun_mask.vtf
            • v_portalgun_normal.vtf
        • w_models/
          • portalgun/
            • w_portalgun.vmt
            • w_portalgun.vtf
            • w_portalgun_exponent.vtf
            • w_portalgun_lightwarp.vtf
            • w_portalgun_normal.vtf
  • models/
    • weapons/
      • V_hands.dx80.vtx
      • V_hands.dx90.vtx
      • v_hands.mdl
      • V_hands.sw.vtx
      • v_hands.vvd
      • v_portalgun.dx80.vtx
      • v_portalgun.dx90.vtx
      • v_portalgun.mdl
      • v_portalgun.sw.vtx
      • v_portalgun.vvd
      • w_portalgun.dx80.vtx
      • w_portalgun.dx90.vtx
      • w_portalgun.mdl
      • w_portalgun.phy
      • w_portalgun.sw.vtx
      • w_portalgun.vvd
  • Read Me.txt
  • sound/
    • weapons/
      • portalgun/
        • portalgun_powerup1.wav
        • portalgun_shoot_blue1.wav
        • portalgun_shoot_red1.wav
        • portal_ambient_loop1.wav
        • portal_close1.wav
        • portal_close2.wav
        • portal_fizzle2.wav
        • portal_invalid_surface3.wav
        • portal_open1.wav
        • portal_open2.wav
        • portal_open3.wav

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