valley vs gully

valley and gully both are nouns.

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

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

valley: a long depression in the surface of the land that usually contains a river
gully: deep ditch cut by running water (especially after a prolonged downpour)

Other hyponyms of valley include dale, glen, holler, hollow, nullah, ravine, rift valley.

valley (noun)	gully (noun)
a long depression in the surface of the land that usually contains a river	deep ditch cut by running water (especially after a prolonged downpour)