hey dude, i just asked my old man whos a mechanic.
i remember u said that ude done ur head before, he says that gunk could b oil from when u done the head. it could have seeped in2 the radiator, because it wasnt flushed out properly when u were gettin it fixed.

he says be careful coz the that amount of gunk could have bloked ur radiator, which means water may b blocked from reachin the gasket, and u may have blown it. that may b y ur losing water aswell.