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

• DBeachball/
  • info.txt
  • materials/
    • dlaor/
      • BeachBallColor.vmt
      • BeachBallColor.vtf
      • BeachBallColor_normal.vtf
  • models/
    • dlaor/
      • DBeachBall.dx80.vtx
      • DBeachBall.dx90.vtx
      • dbeachball.mdl
      • DBeachBall.phy
      • DBeachBall.sw.vtx
      • dbeachball.vvd
      • DBeachBall_huge.dx80.vtx
      • DBeachBall_huge.dx90.vtx
      • dbeachball_huge.mdl
      • DBeachBall_huge.phy
      • DBeachBall_huge.sw.vtx
      • dbeachball_huge.vvd
      • DBeachBall_large.dx80.vtx
      • DBeachBall_large.dx90.vtx
      • dbeachball_large.mdl
      • DBeachBall_large.phy
      • DBeachBall_large.sw.vtx
      • dbeachball_large.vvd
      • DBeachBall_planet.dx80.vtx
      • DBeachBall_planet.dx90.vtx
      • dbeachball_planet.mdl
      • DBeachBall_planet.phy
      • DBeachBall_planet.sw.vtx
      • dbeachball_planet.vvd
      • DBeachBall_small.dx80.vtx
      • DBeachBall_small.dx90.vtx
      • dbeachball_small.mdl
      • DBeachBall_small.phy
      • DBeachBall_small.sw.vtx
      • dbeachball_small.vvd
      • DBeachBall_tiny.dx80.vtx
      • DBeachBall_tiny.dx90.vtx
      • dbeachball_tiny.mdl
      • DBeachBall_tiny.phy
      • DBeachBall_tiny.sw.vtx
      • dbeachball_tiny.vvd
      • DBeachBall_xtralarge.dx80.vtx
      • DBeachBall_xtralarge.dx90.vtx
      • dbeachball_xtralarge.mdl
      • DBeachBall_xtralarge.phy
      • DBeachBall_xtralarge.sw.vtx
      • dbeachball_xtralarge.vvd
  • settings/
    • spawnlist/
      • dlaorsprops.txt
• readmeh.txt

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