gallon vs British capacity unit

gallon and British capacity unit both are nouns.

Word	Noun	Adjective	Verb	Adverb
gallon	Yes	No	No	No
British capacity unit	Yes	No	No	No

As nouns, British capacity unit is a hypernym of gallon; that is, British capacity unit is a word with a broader meaning than gallon:

gallon: a British imperial capacity measure (liquid or dry) equal to 4 quarts or 4.545 liters
British capacity unit: a unit of measure for capacity officially adopted in the British Imperial System; British units are both dry and wet

Other hypernyms of gallon include Imperial capacity unit.

gallon (noun)	British capacity unit (noun)
United States liquid unit equal to 4 quarts or 3.785 liters	a unit of measure for capacity officially adopted in the British Imperial System; British units are both dry and wet
a British imperial capacity measure (liquid or dry) equal to 4 quarts or 4.545 liters