guck vs sludge

guck and sludge both are nouns.

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

As nouns, guck and sludge are synonyms defined as:

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

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