Apart from the period of his military service (brought to an abrupt end by severe wounds) in World War I, Gentle's professional life was spent in the service of his old school; and the influence of his teaching and inspiration on the growth of mathematics and science in Scotland and furth of it was incalculable. His appointment to succeed Dr Clark again in 1926 as Headmaster was a most popular one; and his influence was exerted more tellingly still.
Gentle was, and remained to the day of his death, a man of tremendous energy and vitality, wide in his enthusiasms and restless in pursuit of them. The sole regret of his many friends was that he left no written work.