1930_futurehovercar_final.zip

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
          • 1930 Futurhovercar <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• 1930 Futurhovercar/
  • 1930 futurehovercar 1.6.txt
  • gm_flatgrass0056.jpg
  • gm_flatgrass0058.jpg
  • gm_flatgrass0060.jpg
  • Thumbs.db
  • 1930 Futurhovercar/
    • 1930 futurehovercar 1.6.txt
    • gm_flatgrass0056.jpg
    • gm_flatgrass0058.jpg
    • gm_flatgrass0060.jpg
    • Thumbs.db
  • Futurehovercar information.txt

Kamzo, DapperKat, Juggernaut, oreganazerty, Senses, duck scout, Skefil☆, Grooze, mikhail.plokhov.93, LimeWire314, Ace Kiro, catypollio, DragonKiller (PavelCZ), ✪REKT✪ The Neksi, Nekros, Phoenix_Blaze, Bakari black, Dame, Dame, Opo25, ophktutz, sioncolinwalker, The Scatman, ningken, [S37K] Jackhammer0312, Daniel, Dirth Malet, |Pro| Fatality, Chloe, austin.stokes123, Maple, UncleSHREDDER, |-KPH-|, Alfred Lee Hurst, Archy Bot, Isaac, KaiserMustard, luigi, Uros Uki, Rappy