fall back vs gain

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

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

fall back (noun)	gain (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

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