mallee vs black gum

mallee and black gum both are nouns.

Word	Noun	Adjective	Verb	Adverb
mallee	Yes	No	No	No
black gum	Yes	No	No	No

As nouns, black gum is a hyponym of mallee; that is, black gum is a word with a more specific, narrower meaning than mallee:

mallee: any of several low-growing Australian eucalypts
black gum: a small mallee with rough dark-colored bark toward the butt; yields a red eucalyptus kino gum

mallee (noun)	black gum (noun)
any of several low-growing Australian eucalypts	columnar tree of eastern North America having horizontal limbs and small leaves that emerge late in spring and have brilliant color in early fall
	a small mallee with rough dark-colored bark toward the butt; yields a red eucalyptus kino gum