During the Confederate Heartland Offensive, Maj. Gen. William "Bull" Nelson was wounded in the battle of Richmond and his Army of Kentucky severely mauled. Department commander Horatio G. Wright needed to select a replacement for the wounded Nelson and the two ranking officers, Generals Charles Cruft and James S. Jackson, refused the promotion. Therefore, at the recommendation of both Cruft and Jackson, Wright promoted Gilbert to fill the vacancy, and Gilbert was elevated to acting major general pending the approval of the president. Several days later, on September 9, President Abraham Lincoln promoted Gilbert to brigadier general of U.S. Volunteers. Wright's promotion to acting Major General, however, gave Maj. Gen. Don Carlos Buell enough leverage to appoint Gilbert to a corps command in his Army of the Ohio over such generals as Jeremiah T. Boyle, Jefferson C. Davis, and Albin F. Schoepf. Buell later denied knowing Gilbert had not actually received an official appointment.