when a head gasket fails it normally causes excess pressure in the cooling system and any inperfections within the system will be found.

If the head gasket had failed due to a water problem rather than an oil problem ( most common on 106 ) then the rad may of been leaking just prior which caused the head gasket to blow or as I sugested earlier once the system had suffered excess pressure this weakend a weak rad !

I wouldnt drive with a water leak as this may cause another head gasket to blow, it not just the water level but the pressure in the system is just as important. If you have a leak the system cannot pressurize which will prevent the boiling point of the water rising thus causing water to boil and air pockets at normal driving.