Looks like he was in the 11th Iowa Infantry 9/18/1861-6/14/1865 for the entire war. His regiment was traipsing all over the rest of the country engaging in classic landmark battles of the war--Shiloh, Corinth, Champion Hill, Siege of Vicksburg, Burning of Atlanta, Sherman's March to the Sea--while he apparently spent the duration of the war on detached duty in Missouri.

"COMPANY A
Gardner, Orrin A. A. Age 28. Residence Muscatine, nativity Ohio. Enlisted Sept. 18, 1861. Mustered Sept. 28, 1861. Mustered out June 14, 1865, St. Louis, Mo., expiration of term of service."