gin and tonic vs highball

gin and tonic and highball both are nouns.

Word	Noun	Adjective	Verb	Adverb
gin and tonic	Yes	No	No	No
highball	Yes	No	No	No

As nouns, highball is a hypernym of gin and tonic; that is, highball is a word with a broader meaning than gin and tonic:

gin and tonic: gin and quinine water
highball: a mixed drink made of alcoholic liquor mixed with water or a carbonated beverage and served in a tall glass

gin and tonic (noun)	highball (noun)
gin and quinine water	a mixed drink made of alcoholic liquor mixed with water or a carbonated beverage and served in a tall glass