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
          • FO3 custom camping styled backpack port <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• FO3 custom camping styled backpack port/
  • Materials/
    • Models/
      • Jason278/
        • Fallout 3/
          • 1stpersoncombatknife.vmt
          • 1stpersoncombatknife.vtf
          • 1stpersongrenade.vmt
          • 1stpersongrenade.vtf
          • backpack_black.vmt
          • backpack_black.vtf
          • backpack_desert.vmt
          • backpack_desert.vtf
          • backpack_urban.vmt
          • backpack_urban.vtf
          • backpack_wood.vmt
          • backpack_wood.vtf
          • tex.vmt
          • tex.vtf
  • Models/
    • Fallout 3/
      • Campish_Pack.dx80.vtx
      • Campish_Pack.dx90.vtx
      • campish_pack.mdl
      • Campish_Pack.phy
      • Campish_Pack.sw.vtx
      • campish_pack.vvd
      • Campish_Pack.xbox.vtx

ItzQuantum_vG, Larsess, SpikeBlaze109, _THEON_, Жека Всемогущий, mds, shmfary, terripri819, ℭקĿ ಭĦℑテໂ, sergiomagana2013, banettetejode.yt.2.0, mo3g666, brainsaw_89, manjoon, wardragon.dp, Miojusam, Z4pi, Jude Harley(Komachi), ReverseRetroGamer, misgames69, rap_monster_, Spitz, Mydak11, bellito-bryan, ♥Царь♥, ♥Царь♥, supamasterofdakek, C:\Users\AlphaS>_, Cheyannedavis01, jeykerer, mossesx, indra, sosipisoskuimacev, Lite, FantasyFang22275, edgefighter2017, [KG]AirBorN DC247, [Bsd] Dampi, spartan2064, Kill3rmonk3y, gideonoliveira2, Кристьян, my.little.pony.and.dinosaur, Zavatar93, twenixcz, leσphƣr_, stalkash_, osuruk-kokulubok, datsienko87, antonidragonesgo +168 others