The move will see the 26-year-old join with older brother Craig at Birmingham. Since breaking into the first team in 2011, Gary has spent most of the time away on temporary moves, and Birmingham will be his seventh club on loan.
He spent last season on loan at Barnsley and is set for another spell away from Villa Park. He is already down the pecking order at the club, and his chances will be further limited following the arrival of John McGinn from Hibernian.