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

• murder-master/
  • README.md
  • commands.txt
  • content/
    • data/
      • murder/
        • clue/
          • loot.txt
          • spawns/
            • spawns.txt
        • cs_italy/
          • loot.txt
          • spawns/
            • spawns.txt
        • cs_office/
          • loot.txt
          • spawns/
            • spawns.txt
        • de_chateau/
          • loot.txt
          • spawns/
            • spawns.txt
        • de_dust/
          • loot.txt
          • spawns/
            • spawns.txt
        • de_forest/
          • loot.txt
          • spawns/
            • spawns.txt
        • de_prodigy/
          • loot.txt
          • spawns/
            • spawns.txt
        • de_tides/
          • loot.txt
          • spawns/
            • spawns.txt
        • dm_lockdown/
          • loot.txt
          • spawns/
            • spawns.txt
        • housewithgardenv2/
          • loot.txt
          • spawns/
            • spawns.txt
        • mu_nightmare_church/
          • loot.txt
          • spawns/
            • spawns.txt
        • mu_riverside_b2/
          • loot.txt
        • ttt_clue/
          • loot.txt
          • spawns/
            • spawns.txt
    • materials/
      • murder/
        • melon_logo_scoreboard.png
      • thieves/
        • footprint.vmt
        • footprint.vtf
  • entities/
    • entities/
      • mu_knife/
        • cl_init.lua
        • init.lua
        • shared.lua
      • mu_loot/
        • cl_init.lua
        • init.lua
        • shared.lua
    • weapons/
      • weapon_mu_knife/
        • cl_init.lua
        • init.lua
        • shared.lua
      • weapon_mu_magnum/
        • shared.lua
      • weapon_rp_hands/
        • shared.lua
  • gamemode/
    • cl_adminpanel.lua
    • cl_chattext.lua
    • cl_endroundboard.lua
    • cl_fixplayercolor.lua
    • cl_flashlight.lua
    • cl_footsteps.lua
    • cl_hud.lua
    • cl_init.lua
    • cl_murderer.lua
    • cl_player.lua
    • cl_qmenu.lua
    • cl_ragdoll.lua
    • cl_respawn.lua
    • cl_rounds.lua
    • cl_scoreboard.lua
    • cl_spectate.lua
    • cl_voicepanels.lua
    • init.lua
    • lang/
      • french.lua
      • german.lua
      • italian.lua
      • manee.lua
      • polish.lua
      • portuguese.lua
      • reverse.lua
      • spanish.lua
      • english.lua
    • sh_translate.lua
    • shared.lua
    • sv_adminpanel.lua
    • sv_bystandername.lua
    • sv_chattext.lua
    • sv_flashlight.lua
    • sv_footsteps.lua
    • sv_loot.lua
    • sv_murderer.lua
    • sv_player.lua
    • sv_ragdoll.lua
    • sv_respawn.lua
    • sv_rounds.lua
    • sv_spawns.lua
    • sv_spectate.lua
    • sv_taunt.lua
    • sv_tker.lua
    • weightedrandom.lua
  • icon24.png
  • logo.png
  • murder.fgd
  • murder.txt

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