According to cow
, gack is also the thick, grey, slimy
smelly stuff that builds up in layers
("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.