decay vs gnaw at

decay is a noun but gnaw at is not a noun.

Word	Noun	Adjective	Verb	Adverb
decay	Yes	No	Yes	No
gnaw at	No	No	Yes	No

As verbs, gnaw at is a hyponym of decay; that is, gnaw at is a word with a more specific, narrower meaning than decay:

decay (noun)	gnaw at (noun)
the organic phenomenon of rotting
a gradual decrease; as of stored charge or current
the process of gradually becoming inferior
the spontaneous disintegration of a radioactive substance along with the emission of ionizing radiation
an inferior state resulting from the process of decaying

decay (verb)	gnaw at (verb)
fall into decay or ruin	become ground down or deteriorate
lose a stored charge, magnetic flux, or current
undergo decay or decomposition