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

• Plank Playermodel/
  • icon.tga
  • icon.vtf
  • info.txt
  • lua/
    • autorun/
      • fs_tavo_plank_pm.lua
  • materials/
    • jessev92/
      • player/
        • plank/
          • tavof_texture.vmt
          • tavof_texture.vtf
          • tavo_texture.vmt
          • tavo_texture.vtf
  • models/
    • jessev92/
      • player/
        • misc/
          • plank.dx80.vtx
          • plank.dx90.vtx
          • plank.mdl
          • plank.phy
          • plank.sw.vtx
          • plank.vvd
          • plank.xbox.vtx

PN- DIOSRAYITOFUUU, Anni_MF17, StachuK, sethfirex1750, jakub.majcher2, San :3, cirox.x0, Nemacaco, z3uS.com, leonel.cazaresmurillo, faulnonimus, TheRealLeila, Dominic W, kim.cassin545, lrandrof6, ZorroNXO, blasg5294, Enderloce_Peo64, gabgamers17, Sempai_Chan7v7, Sempai_Chan7v7, ernes7oromero, Dipzi el wena jente, GoddessGriffin, MaffHungry, [TR]kafam_güzel, santoswill1996, Dank_Pool, ElverGalarga, ＰＲＥＣＯＺ., Platina, Gabonzzzio, IsraeLZ, fletcherrobertson060502, Jayfeegle, konohamaruuzumaki, Lineshothellcase.com, gokupowergtyt, ❖ , -x[M]b^ -=Apollo=-, HELL Horror, dbzlover05, asstelevy, Aquarius, Pipepe, Georgeman55, SebaYOLOXDpro, manugomez1997yo, Littlelink612, Alanlion +139 others