evening gown vs dinner dress

evening gown and dinner dress both are nouns.

Word	Noun	Adjective	Verb	Adverb
evening gown	Yes	No	No	No
dinner dress	Yes	No	No	No

As nouns, evening gown and dinner dress are synonyms defined as:

Other synonyms of evening gown include dinner gown, formal.

evening gown (noun)	dinner dress (noun)
a gown for evening wear	a gown for evening wear