gray-headed vs grizzly

gray-headed is not a noun while grizzly is a noun.

Word	Noun	Adjective	Verb	Adverb
gray-headed	No	Yes	No	No
grizzly	Yes	Yes	No	No

As adjectives, gray-headed and grizzly are synonyms defined as:

gray-headed and grizzly: showing characteristics of age, especially having grey or white hair

Other synonyms of gray-headed include gray-haired, gray, grey-haired, grey-headed, grey, hoar, hoary, white-haired.

gray-headed (noun)	grizzly (noun)
	powerful brownish-yellow bear of the uplands of western North America

gray-headed (adjective)	grizzly (adjective)
showing characteristics of age, especially having grey or white hair	showing characteristics of age, especially having grey or white hair