During World War II, Gailer was credited in the destruction of 5.5 enemy aircraft in aerial combat before he was subsequently shot down and taken prisoner.

Flying missions from RAF Leiston, he scored three shared aerial victories in September 1944, with two of them during Operation Market Garden.

Gailer returned to the United States in 1945 and until June 1946, he served as base flight operations officer at Selfridge Field in Michigan.

From there he served as deputy commander for operation of the 3640th Pilot Training Wing at the Laredo Air Force Base in Texas.

In January 1966, he was assigned as the first commander of the newly organized 3630th Flying Training Wing at Sheppard Air Force Base in Texas.

He flew 235 combat missions in the F-100 Super Sabre over South Vietnam and was awarded a second Distinguished Flying Cross for "extraordinary achievement while participating in aerial flight".

[4][1] In September 1969, Gailer arrived at RAF Lakenheath in England, where he assumed command of the 48th Tactical Fighter Wing.

He was named vice commander of the Third Air Force with headquarters at RAF South Ruislip in Middlesex, England, in February 1970.