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
          • George Bush NPC <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• George Bush NPC/
  • info.txt
  • Lua/
    • Autorun/
      • George.lua
  • Materials/
    • Models/
      • gman/
        • gdub_briefcase.vmt
        • gdub_briefcase.vtf
        • gdub_face_map3.vmt
        • gdub_face_map3.vtf
        • gdub_sheet.vmt
        • gdub_sheet.vtf
    • VGUI/
      • Entities/
        • npc_bush.vmt
        • npc_bush.vtf
  • Models/
    • gdub.dx90.vtx
    • gdub.mdl
    • gdub.phy
    • gdub.sw.vtx
    • gdub.vvd
  • Readme.txt

amneisaplayer21, CrazyJason_, banettetejode.yt.2.0, matthewm2230, true gangsta, AUG [TSP], osuruk-kokulubok, alexocedeno, purpleman586, ozi2096, Ry13y04, ChadWarden, Jounzto☭, Adolf Hitler (icejaff), abm2, santiariel22, Fr3d, zennat.3195, timmy999timmy, masterlagartox, 0binar, 0binar, antonio.martins332, Medical By The G, DeadDPpool, huseyin_hamza, [Qc]Nejimmy19, classicgamer43, 3millionaday, Red Panda, COMBINE, Absolutely Arly, Shanker Jr, Hyperweqz, ~~Snorgan~~, BUNNINGS SAUSAGE, baba10110, UncleSHREDDER, MrWARIO (dead), Mr. Wee-a-Boo, luffwafe, Davide Scarlet, B̶u̶b̶b̶l̶e̶s̶ Mr Fisher, Yourakakca, l.maire c, AoDRP I_am_such_a_trollbait, Michooo, sexymacman, Deika, CONRECON