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

• Mammoth Tank/
  • info.txt
  • materials/
    • models/
      • LT_C/
        • Mammoth_Tank/
          • Glass.vmt
          • Mammoth_Tank.vmt
          • Mammoth_Tank.vtf
          • Mammoth_Tank_green.vmt
          • Mammoth_Tank_green.vtf
          • Mammoth_Tank_n.vtf
          • Mammoth_Tank_sand.vmt
          • Mammoth_Tank_sand.vtf
          • Tracks.vmt
  • models/
    • Nirrti/
      • Mammoth_Tank/
        • Mammoth_Tank.dx80.vtx
        • Mammoth_Tank.dx90.vtx
        • mammoth_tank.mdl
        • Mammoth_Tank.phy
        • Mammoth_Tank.sw.vtx
        • mammoth_tank.vvd

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