complainer vs grumbler

complainer and grumbler both are nouns.

Word	Noun	Adjective	Verb	Adverb
complainer	Yes	No	No	No
grumbler	Yes	No	No	No

As nouns, complainer and grumbler are synonyms defined as:

complainer and grumbler: a person given to excessive complaints and crying and whining

Other synonyms of complainer include bellyacher, crybaby, moaner, sniveller, squawker, whiner.

complainer (noun)	grumbler (noun)
a person given to excessive complaints and crying and whining	a person given to excessive complaints and crying and whining