Prosecutors were hoping to get the case to trial by the end of the year.
The new appointment is expected to delay the case 3-to-6 months.
Gallo says his private attorney quit the case, citing a lack of resources.
Investigators say Gallo was drunk when he drove his minivan through a red light in Fullerton and hit another car.
Investigators say Gallo's blood-alcohol level was twice the legal limit, and was driving with a suspended license.
Gallo faces 55 years to life in prison if convicted.
City News Service contributed to this report.