 Gretton was a volunteer officer in the 2nd Volunteer Battalion, The (Prince of Wales's) North Staffordshire Regiment, and served as lieutenant-colonel and colonel when this became the 6th battalion, North Staffordshire Regiment in the Territorial Army from 1907.He was appointed a captain in the Reserve on 24 February 1900.At the outbreak of the First World War he was confirmed as temporary colonel in command of the 6th battalion.