apb.zip

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

• APB/
  • info.txt
  • lua/
    • autorun/
      • client/
        • apb_cl.lua
        • taccloak_cl.lua
      • server/
        • apbcontrol.lua
        • taccloak.lua
      • voices.lua
    • entities/
      • ent_dispatchradio/
        • cl_init.lua
        • init.lua
        • shared.lua
  • materials/
    • sprites/
      • apb.vmt
      • apb.vtf
      • emerg.vmt
      • emerg.vtf
  • sound/
    • charged.wav
    • therm_off.wav
    • therm_on.wav
• README.txt

SirMuerte, ShamanR, statetrooper15, [CMGS]TheGmader[WEB], Fezz, PoE, and Good Brexit to All Men, Bastard Zane the Sage, Polygone Triangle, John Moxley, Rice Cooker, Rice Cooker, Rice Cooker, Rice Cooker, =CCG= PBeezie80, Shadow l TGN, NASA, MitchBruh, Archlich, [TTOE] Cunt punt, Crimson L, Manny | AstraaTv, •Fellix•, Johnson, alecvemmilee24, riley, John Cena, SurrenderMaine, Raptor Jesus, Ricards3, RubenLittle, Noah G. | Tera.GG, Gabe127, [DPA] Irish Quokka, No Name, Faety, Camo, -vG- Miniman0404, mark_xero_04, Suspine, Rilee Czeslaw, Valentino Joan, <SLAUGH7ER>, Phosphene, Rive, Yeke Grumblebeard, Jake T., adwokatPL, Nuf', Eman0039, Sam +5 others