gain vs fall back

gain is a noun but fall back is not a noun.

Word	Noun	Adjective	Verb	Adverb
gain	Yes	No	Yes	No
fall back	No	No	Yes	No

gain (noun)	fall back (noun)
the amount of increase in signal power or voltage or current expressed as the ratio of output to input
the advantageous quality of being beneficial
the amount by which the revenue of a business exceeds its cost of operating
a quantity that is added

gain (verb)	fall back (verb)
increase (one's body weight)	go back to bad behavior
rise in rate or price	retreat
increase or develop	hang (back) or fall (behind) in movement, progress, development, etc.
obtain advantages, such as points, etc.	fall backwards and down
reach a destination, either real or abstract	move back and away from
earn on some commercial or business transaction; earn as salary or wages	have recourse to
win something through one's efforts
derive a benefit from
obtain