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

• de_school/
  • maps/
    • de_school.bsp
    • de_school.txt
    • de_school.nav
  • materials/
    • overviews/
      • de_school_radar.vmt
      • de_school.vmt
      • de_school.vtf
      • de_school_radar.vtf
  • resource/
    • overviews/
      • de_school.txt
  • de_school - readme.txt

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