great gray owl vs great grey owl

great gray owl and great grey owl both are nouns.

Word	Noun	Adjective	Verb	Adverb
great gray owl	Yes	No	No	No
great grey owl	Yes	No	No	No

As nouns, great gray owl and great grey owl are synonyms defined as:

great gray owl and great grey owl: large dish-faced owl of northern North America and western Eurasia

Other synonyms of great gray owl include Strix nebulosa.

great gray owl (noun)	great grey owl (noun)
large dish-faced owl of northern North America and western Eurasia	large dish-faced owl of northern North America and western Eurasia