In mid-June 1863, the IX Corps was reassigned to the Army of the Tennessee. Gerhardt commanded the 46th New York Volunteers as it participated in the Siege of Vicksburg (then nearing its conclusion), and in the Jackson Expedition. The unit participated in the capture of Jackson, Mississippi, on July 17. The IX Corps was transferred to the Army of the Ohio in late July, and Gerhardt moved his troops to the army's headquarters at Knoxville, Tennessee. It participated in the Battle of Blue Springs on October 10, 1863, in Greene County, Tennessee. He was honorably mustered out of the volunteers due to an unspecified disability on November 8, 1863.