put behind bars vs gaol

put behind bars is not a noun while gaol is a noun.

Word	Noun	Adjective	Verb	Adverb
put behind bars	No	No	Yes	No
gaol	Yes	No	Yes	No

As verbs, put behind bars and gaol are synonyms defined as:

Other synonyms of put behind bars include immure, imprison, incarcerate, jail, jug, lag, put away, remand.

put behind bars (noun)	gaol (noun)
	a correctional institution used to detain persons who are in the lawful custody of the government (either accused persons awaiting trial or convicted persons serving a sentence)

put behind bars (verb)	gaol (verb)
lock up or confine, in or as in a jail	lock up or confine, in or as in a jail