01 GMC Sierra 4.3 started missing while driving. I checked the plugs, wires and ignition. I noticed white smoke and water coming out the exhaust, I have had to add coolant a couple of times in the past few months. I don't see any external leaks. Could this be a head gasket. I have got a P0300 & P0302 codes.

Get the engine hot....let it cool down enough that you can attach a coolant pressure tester to the system....pressurize overnight....remove spark plugs, and disable injectors....have someone crank vehicle over and see if coolant comes out of any of the spark plug holes....