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
          • child_worker_m1 <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• child_worker_m1/
  • 09fda6ec.vmt
  • 09fda6ec.vtf
  • 09fda6ec_normal.vtf
  • 24ebf6b5.vmt
  • 24ebf6b5.vtf
  • 24ebf6b5_normal.vtf
  • 5ad436d8.vmt
  • 5ad436d8.vtf
  • 60e1129e.vmt
  • 60e1129e.vtf
  • 60e1129e_normal.vtf
  • 82bf162e.vmt
  • 82bf162e.vtf
  • 82bf162e_normal.vtf
  • 883f2a2e.vmt
  • 883f2a2e.vtf
  • aabc3e37.vmt
  • aabc3e37.vtf
  • aabc3e37_normal.vtf
  • cd948bcf.vmt
  • cd948bcf.vtf
  • cd948bcf_normal.vtf
  • e67afc5c.vmt
  • e67afc5c.vtf
  • e67afc5c_normal.vtf
  • f32306ca.vmt
  • f32306ca.vtf
  • f32306ca_normal.vtf
  • f4c45ac9.vmt
  • f4c45ac9.vtf
  • f4c45ac9_normal.vtf
  • pupil_l.vmt
  • pupil_l.vtf
  • pupil_r.vmt
  • pupil_r.vtf
• Installation.txt

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