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

• Fuckin Creeper/
  • gm_7eleven_v40000.jpg
  • gm_7eleven_v40001.jpg
  • gm_7eleven_v40002.jpg
  • gm_7eleven_v40003.jpg
  • gm_7eleven_v40004.jpg
  • gm_7eleven_v40005.jpg
  • gm_7eleven_v40006.jpg
  • gm_7eleven_v40007.jpg
  • gm_7eleven_v40008.jpg
  • info.txt
  • lua/
    • autorun/
      • fs-dean-the-rapist-playermodel.lua
  • materials/
    • jessev92/
      • dean/
        • creepr/
          • face_diff_002_c_uni.vmt
          • face_diff_002_c_uni.vtf
          • face_normal_002.vtf
          • hair_diff_008_c_uni.vmt
          • hair_diff_008_c_uni.vtf
          • hair_normal_008.vtf
          • head_diff_000_a_whi.vmt
          • head_diff_000_a_whi.vtf
          • head_diff_000_a_whib.vtf
          • uppr_diff_004_a_uni.vmt
          • uppr_diff_004_a_uni.vtf
          • uppr_normal_004.vtf
  • models/
    • jessev92/
      • player/
        • misc/
          • creepr.dx80.vtx
          • creepr.dx90.vtx
          • creepr.mdl
          • creepr.phy
          • creepr.sw.vtx
          • creepr.vvd
          • creepr.xbox.vtx

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