junky vs gym rat

junky and gym rat both are nouns.

Word	Noun	Adjective	Verb	Adverb
junky	Yes	No	No	No
gym rat	Yes	No	No	No

As nouns, gym rat is a hyponym of junky; that is, gym rat is a word with a more specific, narrower meaning than junky:

junky: someone who is so ardently devoted to something that it resembles an addiction
gym rat: someone who spends all leisure time playing sports or working out in a gymnasium or health spa

junky (noun)	gym rat (noun)
a narcotics addict	someone who spends all leisure time playing sports or working out in a gymnasium or health spa
someone who is so ardently devoted to something that it resembles an addiction