gully vs arroyo

gully and arroyo both are nouns.

Word	Noun	Adjective	Verb	Adverb
gully	Yes	No	No	No
arroyo	Yes	No	No	No

As nouns, arroyo is a hyponym of gully; that is, arroyo is a word with a more specific, narrower meaning than gully:

gully: deep ditch cut by running water (especially after a prolonged downpour)
arroyo: a stream or brook

Other hyponyms of gully include draw, wadi.

gully (noun)	arroyo (noun)
deep ditch cut by running water (especially after a prolonged downpour)	a stream or brook