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

• Flood Mod/
  • Flood Mod Most Popular Maps (GD inc. Copyright)/
    • fm_canal_v2.bsp
    • fm_cliffs_v1.bsp
    • fm_oasis2_night.bsp
    • fm_pb_escape.bsp
  • Flood/
    • entities/
      • weapons/
        • gmod_tool/
          • cl_init.lua
          • cl_viewscreen.lua
          • ghostentity.lua
          • init.lua
          • object.lua
          • shared.lua
          • stool.lua
          • stool_cl.lua
          • stools/
            • axis.lua
            • balloon.lua
            • ballsocket.lua
            • ballsocket_adv.lua
            • ballsocket_ez.lua
            • button.lua
            • camera.lua
            • colour.lua
            • duplicator.lua
            • dynamite.lua
            • elastic.lua
            • emitter.lua
            • example.lua
            • eyeposer.lua
            • faceposer.lua
            • finger.lua
            • hoverball.lua
            • hydraulic.lua
            • ignite.lua
            • inflator.lua
            • keepupright.lua
            • lamp.lua
            • leafblower.lua
            • light.lua
            • magnetise.lua
            • material.lua
            • motor.lua
            • muscle.lua
            • nail.lua
            • nocollide.lua
            • paint.lua
            • physprop.lua
            • prop_healer.lua
            • prop_remover.lua
            • pulley.lua
            • remover.lua
            • rope.lua
            • rtcamera.lua
            • slider.lua
            • spawner.lua
            • statue.lua
            • thruster.lua
            • trails.lua
            • turret.lua
            • vgui/
              • duplicator_gui.lua
              • duplicator_gui_button.lua
            • weld.lua
            • weld_ez.lua
            • wheel.lua
            • winch.lua
    • gamemode/
      • cl_HUDS.lua
      • cl_init.lua
      • cl_Menu.lua
      • cl_Score.lua
      • CS.lua
      • init.lua
      • Players.lua
      • PP.lua
      • shared.lua
    • info.txt
  • Information on how to install corectly..txt

jiwang.kim0707, Nikko, sofiann.agounine, ℙ︎Ξ┬︎∁︎α︎ℕ@, Stella Nova, fourQ, kauan.modesto.10, ValenGraff11, GROSH, Lord Habichuela, Hu_ya, axerkiller, eunha2006, 스파웃퍼, 신웅, chara_youtube, vizanti, themsnjosue, alexekvz, AGamumung, koslov1943, kauefmota7, tonytoce03, mes0072, [KOR]수박맛과자, 말링, 라이, Wiktoreq, Szysz, Mr2468601, ZacGames, Tou, 机械拉枪, ma641088, noriegaariel83, noriegaariel83, KimYeoJin, FLAIN, DDO-E, Alucard, Hotaro, Skd, LOSvenomwolf, MrProof [Uranium], MrProof [Uranium], O/ This is Bob, G2 Team KennyS, abd.omari99, [kor]YS, WolfKing +736 others