gym rat vs junky

gym rat and junky both are nouns.

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

As nouns, junky is a hypernym of gym rat; that is, junky is a word with a broader meaning than gym rat:

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

Other hypernyms of gym rat include addict, freak, junkie, nut.

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