During the war, he commanded Battery C, 3rd U.S. Artillery, and was part of the famed U.S. Horse Artillery Brigade in the Army of the Potomac. Cited for gallantry, he was awarded brevet (honorary) promotions to major (May 5, 1862, for actions at Williamsburg) and lieutenant colonel (September 17, 1862, for actions at Antietam). By 1863, he accepted a commission in the U.S. Volunteers and commanded the 2nd Ohio Artillery as a lieutenant colonel and colonel. At the end of the war, Gibson was awarded a brevet appointment as colonel in the Regular Army. On January 13, 1866, President Andrew Johnson nominated Gibson for appointment to the grade of brevet brigadier general of volunteers, to rank from March 13, 1865, and the United States Senate confirmed the appointment on March 12, 1866.