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
          • Best Tools Pack <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• Best Tools Pack/
  • All Best tools.txt
  • data/
    • smartsnap_offsets_default.txt
  • info.txt
  • lua/
    • autorun/
      • client.lua
      • laserlib.lua
      • server/
        • sv_fin.lua
    • entities/
      • fin_2/
        • cl_init.lua
        • init.lua
        • shared.lua
      • gmod_commandbox/
        • cl_init.lua
        • init.lua
        • shared.lua
      • gmod_fan/
        • cl_init.lua
        • init.lua
        • shared.lua
      • gmod_laser/
        • cl_init.lua
        • init.lua
        • shared.lua
      • gmod_laser_reflector/
        • cl_init.lua
        • init.lua
        • shared.lua
      • gmod_perimeterturret/
        • cl_init.lua
        • init.lua
        • shared.lua
      • gmod_perimeterturret_rocket/
        • cl_init.lua
        • init.lua
        • shared.lua
      • gmod_pressurepad/
        • cl_init.lua
        • init.lua
        • shared.lua
      • gmod_radio/
        • cl_init.lua
        • init.lua
        • shared.lua
      • gmod_sensor/
        • cl_init.lua
        • init.lua
        • shared.lua
      • gmod_timer/
        • cl_init.lua
        • init.lua
        • shared.lua
      • gmod_toggleablehoverball/
        • cl_init.lua
        • init.lua
        • shared.lua
      • gmod_wire_buoyancy/
        • cl_init.lua
        • init.lua
        • shared.lua
      • malawar_airbrake/
        • cl_init.lua
        • init.lua
        • shared.lua
      • malawar_cannon/
        • cl_init.lua
        • init.lua
        • shared.lua
      • malawar_cannonround/
        • cl_init.lua
        • init.lua
        • shared.lua
      • malawar_harpoon/
        • cl_init.lua
        • init.lua
        • shared.lua
      • malawar_harpoon_launcher/
        • cl_init.lua
        • init.lua
        • shared.lua
      • malawar_heatseeker/
        • cl_init.lua
        • init.lua
        • shared.lua
      • malawar_launcher/
        • cl_init.lua
        • init.lua
        • shared.lua
      • malawar_repulsor/
        • cl_init.lua
        • init.lua
        • shared.lua
      • malawar_rocket/
        • cl_init.lua
        • init.lua
        • shared.lua
      • malawar_torpedo/
        • cl_init.lua
        • init.lua
        • shared.lua
      • malawar_xcannon/
        • cl_init.lua
        • init.lua
        • shared.lua
      • malawar_xcannonround/
        • cl_init.lua
        • init.lua
        • shared.lua
      • particle_sent/
        • cl_init.lua
        • init.lua
        • shared.lua
      • sent_keypad/
        • cl_init.lua
        • init.lua
        • shared.lua
      • sent_keypad_wire/
        • cl_init.lua
        • init.lua
        • shared.lua
    • PerimeterTurrets/
      • Laser.lua
      • Rocket.lua
      • Shotgun.lua
      • SMG.lua
    • weapons/
      • gmod_tool/
        • stools/
          • airbrake.lua
          • breakableprops.lua
          • cannon.lua
          • cloaking.lua
          • commandbox.lua
          • constraintbreaker.lua
          • door.lua
          • easy_precision.lua
          • ez_slider.lua
          • fadingdoor.lua
          • fan.lua
          • fin2.lua
          • harpoon.lua
          • keypad.lua
          • keypad_adv.lua
          • keypad_wire.lua
          • laseremitter.lua
          • launcher.lua
          • nocollide_world.lua
          • no_collide_multi.lua
          • ol_measuringstick.lua
          • ol_stacker.lua
          • particlesystem.lua
          • perimeterturret.lua
          • positionswitcher.lua
          • pressurepad.lua
          • repulsor.lua
          • rotator.lua
          • rt_buoyancy.lua
          • rt_buoyancy_wire.lua
          • sensor.lua
          • smartwelder.lua
          • timer.lua
          • toggleablehoverballs.lua
          • unbreakable.lua
          • weight.lua
          • wing.lua
          • xcannon.lua
  • materials/
    • VGUI/
      • entities/
        • gmod_laser_killicon.vmt
        • gmod_laser_killicon.vtf
        • gmod_laser_reflector.vmt
        • gmod_laser_reflector.vtf
  • models/
    • madjawa/
      • laser_reflector.dx80.vtx
      • laser_reflector.dx90.vtx
      • laser_reflector.mdl
      • laser_reflector.phy
      • laser_reflector.sw.vtx
      • laser_reflector.vvd

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