The Giftspritze

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
          • TF2 The Giftspritze hexed <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• TF2 The Giftspritze hexed/
  • materials/
    • models/
      • weapons/
        • c_items/
          • germgun.vmt
          • germgun.vtf
  • models/
    • Gifpons/
      • c_models/
        • c_leechgun/
          • c_leechgun.dx80.vtx
          • c_leechgun.dx90.vtx
          • c_leechgun.mdl
          • c_leechgun.phy
          • c_leechgun.sw.vtx
          • c_leechgun.vvd
          • c_leechgun.xbox.vtx

Ashir PL, LambdaKoopa, DR.Enginier[PL], Boogaclaw, ☭ Lord TL ☭, DecoyofChivalry, Warpath-the-drone swe, The Imperial Combine, captsnow, The Rocket Gamer, Di`Kut, Dis Dolphin Guy.., Damn Daniel, PeculiarPsychopath