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
          • Half-Life Apache Snpc <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• Half-Life Apache Snpc/
  • info.txt
  • lua/
    • autorun/
      • thenpcmenu.lua
    • entities/
      • npc_apache2/
        • cl_init.lua
        • init.lua
        • shared.lua
      • proj_dan_heli_shot/
        • cl_init.lua
        • init.lua
        • shared.lua
  • sound/
    • apache/
      • fire.wav

mw, nicolastobiasquinterosperdomo, Rigit10, Rigit10, ismet.balic97, peeterxxx2, _FLUX_, Hullu Hapua, Retroxturn, Tempo_Games, danya.raxmonov.07, Q6_m, kiapekenmika, madi pas, oussamaiounouss, iaiasan1964, hunter2000_, magun00009, maxusa64, maxusa64, panathonpolsit2548, Nihilanth, Nihilanth, BurakProGh, BurakProGh, AquaHx, Tal Rex, braiancondorinqn1, braiancondorinqn1, tuputamadre2006, OGYR4IK, buborek2003 [GameTame.com], lino136gr, chet98, 2740996942, Bigsmoke Fat boi, misgames69, XeNTIPeDe, szabiszabika3 hellcase.com, loiloveyou424, SkyCraftProGamer, mohabthehero, oskarwysocki618, Ari_Takeda, mardutajayarocks, tiger174, mimod17, mimod17, szalaiakos05, ibrahimkral4444 +357 others