I have a 2003 monte carlo as 3.8 pace car. There are only 5 cylinders firing. I recently has to replace the head gasket and other gaskets. Any suggestions?

 

There are too many possible causes of a misfire to provide any real help without some diagnostic info.

First step needs to be identifying which cylinder (scan tool, looking at the plugs, measuring exhaust runner temps).

From there you can start inspecting/ testing components related to that cylinder. If you don't have tools to inspect everything, some parts can be swapped around to see if the issue moves - injector, plug, wire, coil pack.

 

Could be a bad crank sensor, bad coils, or a bad ignition module. Could even be bad injectors, but not usually if more than 1 cylinder is misfiring. I recently drove through a big puddle and my 2002 SS started to run like crap and I got a misfire code. I think it was the crank sensor connector got wet. Later that day it was fine.