Return to UK, marriage & 1914
From 1908 until he rejoined his own regiment in 1910, Gethin was adjutant for the 3rd Battalion, The North Nigerian Regiment He was promoted Captain on 9 September 1909 and that same year his brother Hugh began his Civil Service career in Nigeria.
Gethin spent a period in London as Captain of the King's Guards at St. James's Palace, prior to his marriage on 29 September 1910, at Holy Trinity Church, Abergavenny, to Ethel Berry Walford (b 24 October 1878), daughter of solicitor James B. Walford of Penypound Chapel House, Abergavenny. Their only child, John Gethin Morgan-Owen was born on 22 August 1914.
Early in August 1914 Germany had declared war on Russia, then on France and invaded both Luxembourg and Belgium. Britain declared war on Germany on 4 August and on Austria on 12 August. At the outbreak of war Gethin was at Staff College. From 5 August until 16 September he was Assistant Embarkation Staff Officer (graded Staff Capt.) at Southampton. Dispatching men, supplies and equipment was a major logistical exercise; the first of the British Expeditionary Force landed in France on 17 August. On 17 September he was appointed brigade major of 40th Brigade, 13th Division of the New Armies, with whom he would spend the rest of the war, mainly engaged in fighting the Turks. The 40th Brigade consisted of 8th Battalion the Cheshire Regiment, 8th Battalion the Royal Welsh Fusiliers and 4th Battalion the South Wales Borderers, in which his brother Gurth served. From August 1914 to January 1915 it also included 8th Battalion the Welsh Regiment and, from December 1915, 5th Battalion the Wiltshires.