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
          • Drivable Nissan Skyline R34 by TheDanishMaster <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• Drivable Nissan Skyline R34 by TheDanishMaster/
  • materials/
    • models/
      • tdmcars/
        • r34/
          • skin2.vmt
          • skin2.vtf

bluevelocitygaming, klukasz196, leonard.antuness, STALKER2, rnhb, toumou.roy, erikvillam, BlinkarN, ObiWan, s △ d, Träger/ faster than you!, kvneki.kids, asriel, General Mostafa, Supreme Kaii, svenda.aron0901, POMPON 123, 2hard2dodge, deep, ♕MilanTheMLGPro♕, farkas, enesege213, E}|{Иd3e, CRimZ, H1R0™ -GCM-, I.J.U.S.T.I.C.E, Rokimin, Momo1000000k, kooser, belizariosilvad, benilol, zsgames32, bastv556677, alejandry040, davide24826, nukeishonzamo, ☜✯☞FORZA☜✯☞, soulohio, Dillion Harper, ✪ kizz, utcewa, eduardo24perez24, 4thg4, Red_Bob, nikita.struts, mossesx, dastreop, Sajtos Kutya, Пейган Мин, YTBluu +674 others