During the American Civil War, he enlisted in the state's Emergency Volunteer Militia as a private in Landis' Philadelphia Battery at the time of the Robert E. Lee's 1863 invasion of Pennsylvania. After the Confederates were defeated in the Battle of Gettysburg, Gilder and his unit were mustered out in August. The death of his father, while serving as chaplain of the Fortieth New York Volunteers, obliged him to give up the study of the law.