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

• GMStranded/
  • entities/
    • effects/
      • GMS_LootEffect/
        • init.lua
    • entities/
      • GMS_AntlionBarrow/
        • cl_init.lua
        • init.lua
        • shared.lua
      • GMS_Buildsite/
        • cl_init.lua
        • init.lua
        • shared.lua
      • GMS_Food/
        • cl_init.lua
        • init.lua
        • shared.lua
      • GMS_Furnace/
        • cl_init.lua
        • init.lua
        • shared.lua
      • GMS_Grindingstone/
        • cl_init.lua
        • init.lua
        • shared.lua
      • GMS_Loot/
        • cl_init.lua
        • init.lua
        • shared.lua
      • GMS_Mail/
        • cl_init.lua
        • init.lua
        • shared.lua
      • GMS_ResourceDrop/
        • cl_init.lua
        • init.lua
        • shared.lua
      • GMS_Seed/
        • cl_init.lua
        • init.lua
        • shared.lua
      • GMS_Stove/
        • cl_init.lua
        • init.lua
        • shared.lua
      • GMS_WaterFountain/
        • cl_init.lua
        • init.lua
        • shared.lua
      • GMS_Well/
        • cl_init.lua
        • init.lua
        • shared.lua
      • GMS_Workbench/
        • cl_init.lua
        • init.lua
        • shared.lua
    • weapons/
      • gmod_tool/
        • stools/
          • ignite.lua
          • rope.lua
      • gms_fishingrod/
        • shared.lua
      • gms_fryingpan/
        • shared.lua
      • gms_hands/
        • shared.lua
      • gms_hatchet/
        • shared.lua
      • gms_pickaxe/
        • shared.lua
      • gms_shovel/
        • shared.lua
      • gms_sickle/
        • shared.lua
      • gms_spoon/
        • shared.lua
      • gms_strainer/
        • shared.lua
  • gamemode/
    • chatcommands.lua
    • cl_init.lua
    • cl_panels.lua
    • cl_spawnmenu.lua
    • combinations.lua
    • init.lua
    • processes.lua
    • shared.lua
    • unlocks.lua
    • _cl_scoreboard.lua
  • help.htm
  • help.htm.ztmp
  • info.txt

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