That'd require a whole lot of modding.
Sound files (Real gunshots), replaced models (Shotgun -> Spudgun), Particles (Blood), Animations (Real death anims like GTA IV/Call of Duty), scripting (Make sure they don't spawn again) & Trouble meter hacking.
Or you'd have GTA: Bully
Which then means you'd made Bully into GTA while I made GTA into Bully.... weird. O_o