grapefruit vs citrus fruit

grapefruit and citrus fruit both are nouns.

Word	Noun	Adjective	Verb	Adverb
grapefruit	Yes	No	No	No
citrus fruit	Yes	No	No	No

As nouns, citrus fruit is a hypernym of grapefruit; that is, citrus fruit is a word with a broader meaning than grapefruit:

grapefruit: large yellow fruit with somewhat acid juicy pulp; usual serving consists of a half
citrus fruit: any of numerous fruits of the genus Citrus having thick rind and juicy pulp; grown in warm regions

Other hypernyms of grapefruit include citrous fruit, citrus.

grapefruit (noun)	citrus fruit (noun)
large yellow fruit with somewhat acid juicy pulp; usual serving consists of a half	any of numerous fruits of the genus Citrus having thick rind and juicy pulp; grown in warm regions
citrus tree bearing large round edible fruit having a thick yellow rind and juicy somewhat acid pulp