According to
cow of
#e, gack is also the thick, grey,
slimy smelly stuff that builds up in
layers and
clumps ("thick, juicy clumps", even, in her words) in
musical instruments, particularly
brass ones. Apparently it's good to clean your instruments often and thoroughly...
Nickelodeon also once marketed a slime-like product under the name of Gak(tm), though I don't know if that has anything to do with cow's choice of nomenclature.