sludge vs gunk

sludge and gunk both are nouns.

Word	Noun	Adjective	Verb	Adverb
sludge	Yes	No	No	No
gunk	Yes	No	No	No

As nouns, sludge and gunk are synonyms defined as:

Other synonyms of sludge include goo, gook, goop, guck, muck, ooze, slime.

sludge (noun)	gunk (noun)
any thick, viscous matter	any thick, viscous matter
the precipitate produced by sewage treatment