gaol vs put behind bars

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

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

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

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

gaol (noun)	put behind bars (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)

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