sun gear vs gear wheel

sun gear and gear wheel both are nouns.

Word	Noun	Adjective	Verb	Adverb
sun gear	Yes	No	No	No
gear wheel	Yes	No	No	No

As nouns, gear wheel is a hypernym of sun gear; that is, gear wheel is a word with a broader meaning than sun gear:

sun gear: the central gear in an epicyclic train
gear wheel: a toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion

Other hypernyms of sun gear include cogwheel, gear, geared wheel.

sun gear (noun)	gear wheel (noun)
the central gear in an epicyclic train	a toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion