The mod adds a whole bunch of zombies that will try to beat you to death.
Extract the contents to GTA2\data and run it with the GTA2 manager in the Debug tab, typing GMP = zombies.gmp, STY = ste.sty, SCR = zombies.scr like in the picture above.
You can also start by creating a multiplayer game in GTA2 Game Hunter, choose a zombie map and play it alone.