planet gear vs gear wheel

planet gear and gear wheel both are nouns.

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

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

planet gear: an outer gear that revolves about a central sun gear of 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 planet gear include cogwheel, gear, geared wheel.

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