radioactivity unit vs Gy

radioactivity unit and Gy both are nouns.

Word	Noun	Adjective	Verb	Adverb
radioactivity unit	Yes	No	No	No
Gy	Yes	No	No	No

As nouns, Gy is a hyponym of radioactivity unit; that is, Gy is a word with a more specific, narrower meaning than radioactivity unit:

radioactivity unit: a measure of radioactivity
Gy: the SI unit of energy absorbed from ionizing radiation; equal to the absorption of one joule of radiation energy by one kilogram of matter; one gray equals 100 rad

Other hyponyms of radioactivity unit include millicurie, Ci, curie, gray, R, roentgen, rutherford, rem, millirem, mrem, rad, milliroentgen.

radioactivity unit (noun)	Gy (noun)
a measure of radioactivity	the SI unit of energy absorbed from ionizing radiation; equal to the absorption of one joule of radiation energy by one kilogram of matter; one gray equals 100 rad