The gunk is more than likely condensation and forming this white gunk, I would change the oil clean out the gunk and make sure the car is given a long run now and then to burn off any condensation forming To check the head gasket has failed is the engine loosing water, if so repair any water leaks first possible replace radiator cap this will ensure the cooling system is capped correctly and no water leaks from the header tank, now if the cooling system is pressurising this exhaust gages escaping from the cylinder and pressurising the coolant system to check for pressure fill with the engine ticking over slowly fill the header tank to the top until it over flows does the water bubble up if it does the head gasket has gone you can buy a head gasket test kit from Halfords.
Alan, May 2012
