The Collaborative International Dictionary of English v.0.48:

gunk \gunk\ n.
   any thick gooey and messy substance. [informal]

   Syn: goo, gook, guck, muck, ooze, sludge, slime.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

gunk
    n 1: any thick, viscous matter [syn: sludge, slime, goo,
         goop, gook, guck, gunk, muck, ooze]