The 99th, known as the "Union Coast Guard", or "Bartlett's Naval Brigade", was initially organized by Colonel William A. Bartlett in New York City, though it included many men from Massachusetts. George served one year and mustered out of the 99th Infantry in August 1862, volunteering again with the 4th Massachusetts Cavalry, where he served as band leader through 1865. Campaigns engaged by this unit during Gewinner's time include; Appomattox Campaign, 3rd Battle of Petersburg, Battle of High Bridge, and the Battle of Appomattox Courthouse. It was reported that Gewinner's band played at the surrender of General Robert E. Lee to General Ulysses S. Grant at the McLean House near Appomattox Courthouse on April 9, 1865. A few months after the surrender, George mustered out of service again in Boston, Massachusetts Sep 1865.