If it was your head gasket, then a compression test would not necesarily indicate anything. It depends on where the failure is. If it's between an oil gallery and the water jacket, then your compressions will not be affected. If the failure is between the water jacket or oil gallery and a combustion chamber, then you will get a low compression on the affected cylinder. If the failure is between two cylinders, then you will have low compressions on both affected cylinders.