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
          • Gjail 1.3 <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• Gjail 1.3/
  • data/
    • gjail/
      • ba_jail_mars.txt
  • gamemode/
    • gjail 1.3/
      • entities/
        • entities/
          • ent_knife/
            • cl_init.lua
            • init.lua
            • shared.lua
          • weapon_ent_flashbang/
            • cl_init.lua
            • init.lua
            • shared.lua
        • weapons/
          • css_knife/
            • shared.lua
          • fists/
            • shared.lua
          • scanner/
            • shared.lua
          • weapon_akc/
            • shared.lua
          • weapon_aug_rifle/
            • shared.lua
          • weapon_deaglec/
            • shared.lua
          • weapon_deagle_gun/
            • shared.lua
          • weapon_disguise/
            • shared.lua
          • weapon_drug_syringe/
            • shared.lua
          • weapon_duel_USPs/
            • shared.lua
          • weapon_famasc/
            • shared.lua
          • weapon_famas_rifle/
            • shared.lua
          • weapon_fist/
            • shared.lua
          • weapon_flashbang/
            • shared.lua
          • weapon_guard_gun/
            • shared.lua
          • weapon_hands/
            • shared.lua
          • weapon_knife/
            • shared.lua
          • weapon_m4a1_rifle/
            • shared.lua
          • weapon_m4c/
            • shared.lua
          • weapon_mc/
            • shared.lua
          • weapon_mp5c/
            • shared.lua
          • weapon_pickpocket/
            • shared.lua
          • weapon_prisoner_gun/
            • shared.lua
          • weapon_pumpshotgunc/
            • shared.lua
          • weapon_real_base/
            • shared.lua
          • weapon_real_base_pistol/
            • shared.lua
          • weapon_real_base_reddot/
            • cl_init.lua
            • shared.lua
          • weapon_real_base_rifle/
            • shared.lua
          • weapon_real_base_smg/
            • shared.lua
          • weapon_real_base_snip/
            • cl_init.lua
            • shared.lua
          • weapon_real_base_special_aim/
            • shared.lua
          • weapon_scoutc/
            • shared.lua
          • weapon_stmpc/
            • shared.lua
          • weapon_tranquilizer/
            • shared.lua
          • weapon_usp2c/
            • shared.lua
          • weapon_uspc/
            • shared.lua
      • gamemode/
        • cl_command.lua
        • cl_init.lua
        • init.lua
        • shared.lua
        • sv_admin.lua
        • sv_client.lua
        • sv_rounds.lua
        • sv_tables.lua
  • maps/
    • ba_jail_mars.bsp

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