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

• Screenshots/
  • zmrp_smalltown0000.jpg
  • zmrp_smalltown0001.jpg
  • zmrp_smalltown0002.jpg
  • zmrp_smalltown0003.jpg
  • zmrp_smalltown0004.jpg
  • zmrp_smalltown0005.jpg
  • zmrp_smalltown0006.jpg
  • zmrp_smalltown0007.jpg
  • zmrp_smalltown0008.jpg
  • zmrp_smalltown0009.jpg
  • zmrp_smalltown0010.jpg
• READ-ME.txt
• zmrp_smalltown_fixed.bsp

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