ok this may seem crazy, but soak it in coke cola over night. The cola will dissolve a nail in a matter of days, it may be enough to break up that gunk. It also works to leave old flat coke in the toilet as tolet bowl cleaner FYI. The sugar residue is a whole lot easier to clean out than that gunk.
Annie, Annie, Annie! Tsk,Tsk,Tsk! :lol: