angel vs guardian angel

angel and guardian angel both are nouns.

Word	Noun	Adjective	Verb	Adverb
angel	Yes	No	No	No
guardian angel	Yes	No	No	No

As nouns, guardian angel is a hyponym of angel; that is, guardian angel is a word with a more specific, narrower meaning than angel:

angel: spiritual being attendant upon God
guardian angel: an angel believed to have special affection for a particular individual

Other hyponyms of angel include archangel, cherub, seraph, guardian spirit, divine messenger.

angel (noun)	guardian angel (noun)
spiritual being attendant upon God	an angel believed to have special affection for a particular individual
someone who provides financial support for some venture
person of exceptional holiness