gray matter vs nerve tissue

gray matter and nerve tissue both are nouns.

Word	Noun	Adjective	Verb	Adverb
gray matter	Yes	No	No	No
nerve tissue	Yes	No	No	No

As nouns, nerve tissue is a hypernym of gray matter; that is, nerve tissue is a word with a broader meaning than gray matter:

gray matter: greyish nervous tissue containing cell bodies as well as fibers; forms the cerebral cortex consisting of unmyelinated neurons
nerve tissue: tissue composed of neurons

Other hypernyms of gray matter include nervous tissue.

gray matter (noun)	nerve tissue (noun)
greyish nervous tissue containing cell bodies as well as fibers; forms the cerebral cortex consisting of unmyelinated neurons	tissue composed of neurons