gunnel vs rock gunnel

gunnel and rock gunnel both are nouns.

Word	Noun	Adjective	Verb	Adverb
gunnel	Yes	No	No	No
rock gunnel	Yes	No	No	No

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

gunnel: small eellike fishes common in shallow waters of the northern Atlantic
rock gunnel: slippery scaleless food fish of the northern Atlantic coastal waters

Other hyponyms of gunnel include butterfish, Pholis gunnellus.

gunnel (noun)	rock gunnel (noun)
small eellike fishes common in shallow waters of the northern Atlantic	slippery scaleless food fish of the northern Atlantic coastal waters
wale at the top of the side of boat; topmost planking of a wooden vessel