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
          • advdupe2 <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• advdupe2/
  • cannon and turret/
    • flak38_12.txt
    • m1a1.txt
    • m224.txt
    • M2A1 Final.txt
    • m3 cannon.txt
    • m3.txt
    • maching.txt
    • mc gun.txt
    • old acf gun.txt
    • reloadcaooon.txt
    • sadsad.txt
    • supergun.txt
    • test.txt
    • vulcan.txt
    • vulcan_1.txt
  • car/
    • electric car.txt
    • frame.txt
    • JEEP_9.txt
    • maz535_1.txt
    • offroad blue.txt
    • offroad red light.txt
    • offroad red.txt
    • offroad.txt
    • tractor_9.txt
    • tricar_8.txt
    • turbo car.txt
    • turbo offroad.txt
  • dupereplace/
    • flak car.txt
    • pickupcar.txt
    • somany.txt
    • tank1.txt
    • zgd.txt
  • engine/
    • i10.txt
    • singlediesel.txt
  • enginecar/
    • engine car b4.txt
    • engine car b6.txt
    • engine car i3.txt
    • engine car r10.txt
    • engine car r5.txt
    • engine car v8.txt
    • engine car v8_1.txt
    • engine car v8_2.txt
    • engine offroad.txt
    • engine tank.txt
    • engine tank_1.txt
    • enginepickup.txt
    • new engine.txt
    • old engine car.txt
    • test tank.txt
    • v8 at.txt
  • golf/
    • VW Golf MK2 4WD.txt
    • VW Golf MK2.txt
  • helicopter/
    • ch47 test.txt
    • heli test.txt
    • helicopter test.txt
    • helicopter test_1.txt
    • R22_3.txt
  • lotus98t/
    • lotus 98t grip.txt
    • lotus 98t.txt
  • m3new/
    • M16 Green.txt
    • m55.txt
    • M55_1.txt
  • minicar/
    • mini car blue.txt
    • mini tractor red.txt
  • ship/
    • ACF Warship - Cruiser.txt
    • desmoines lowlag.txt
    • desmoines.txt
    • DHG.txt
    • tenryu final.txt
    • tenryu.txt
    • test battle ship.txt
    • test boat.txt
  • suspension/
    • axle.txt
    • Christie.txt
    • none.txt
  • t54/
    • SU140 Green.txt
    • T54 1951 Green.txt
    • Type59 Green.txt
  • t55a/
    • BTRT Desert.txt
    • T55 Desert.txt
    • T55 Green.txt
    • T55 tiger.txt
    • T55A Desert.txt
    • T55A Green.txt
    • ZSU76.txt
  • t62/
    • T62 Desert.txt
    • T62 Green.txt
    • T62 newsound.txt
  • t95new/
    • light.txt
    • T28 Backup.txt
    • T28 Grass.txt
    • T28 Green.txt
    • t28.txt
    • T95 detail.txt
    • T95 Green.txt
    • t95.txt
    • t95new.txt
    • t95_1.txt
    • t95_10.txt
    • t95_11.txt
    • t95_12.txt
    • t95_13.txt
    • t95_14.txt
    • t95_15.txt
    • t95_16.txt
    • t95_17.txt
    • t95_18.txt
    • t95_19.txt
    • t95_2.txt
    • t95_3.txt
    • t95_4.txt
    • t95_5.txt
    • t95_6.txt
    • t95_7.txt
    • t95_8.txt
    • t95_9.txt
  • tank/
    • FV4005 Green.txt
    • IS4M Green.txt
    • kv6 Green.txt
    • M18 Green2.txt
    • M551 Sheridan.txt
    • M551test.txt
    • Small tank_4.txt
    • T34 Green.txt
    • TKS 762 2 seat Green.txt
  • tankai/
    • Tank Cannoneer German.txt
    • Tank Cannoneer.txt
    • tank driver german.txt
    • tank driver.txt
    • tank driver2 german.txt
    • tank driver2.txt
  • tanknpc/
    • luchs.txt
    • m4a1.txt
    • m4a1npc.txt
    • m4a1_1.txt
    • m4a1_2.txt
    • pz4lowprop.txt
    • pz4npc.txt
    • tiger2.txt
  • tankrace/
    • rac.txt
    • rac_1.txt
    • rac_2.txt
  • type94truck/
    • Type 94 AA Green.txt
    • Type 94 Green.txt
  • type95/
    • Type95 Green Suspension.txt
    • Type95 Green.txt
    • Type95 PLA.txt
    • Type95 Red.txt

专业想坑队友1000000000000000000000000年, 1545361775, SlavicBoi, SowsemNeVor, ermiakhan.new, -BloodyEmpire-Echo, Dazer, dekorob962, Ahmad, foxymaster2, LowerPlayz, Иоська, Mif200k, Chris Reynolds, Rutex, Mola [MASTER], [BWA] Satanael, [...]Дезертир, _FiLin_, Peter "Oh shit" Davison, Piston, Retko, BAD Gicsich, Stomatolog505ALXRP, salimplayz8, Berserk, erikvillam, sensimilla420, ♥ {-MaiDen-} ♥, Gaster.ink, [TAF] Swat07, pick.papa, lgml067gp, ☭Lagio☭, ☭Lagio☭, <|SYP|> An FBI Man, ☭ท่านเปา♥, Kenzy, ludshiy00, wiphoow5, 백만볼트, Lauren_Pasc, Meme Stalin, Sammelcheks, Yunofeedme24/7, BlueStarFox, Маслину поймал, 닉네임없음, brendon8ga, CroSs +1483 others