A blown head gasket is not the only reason you could be losing water, may sound obvious but have you eliminated the water pump, thermostat housing,radiator,and all hoses and connections. When you stop the car are there any signs of water dripping? Why did you suspect it was the head gasket? Oil/water contamination, poor running/misfire over heating etc? Did you check the cylinder head had no cracks in it when you took it off and did you check it was not warped? If the cylinder head is not completely flat a new head gasket will make no difference.If I am telling you things you have already checked I am sorry but not everyone thinks of all the possible reasons.