Smallwood joined the Royal Warwickshire Regiment in 1911 as a Special Reserve second lieutenant on probation, confirmed in April 1912. In December 1912, after passing his examination, he was transferred to be a regular second lieutenant in the 2nd battalion, East Yorkshire Regiment. In September 1914, after the outbreak of World War I, he was promoted to temporary lieutenant and later that year seconded to the Army Signal Service. He was promoted to captain in 1915. He was twice mentioned in dispatches during the war, and he was awarded the Military Cross in the King's 1916 Birthday Honours. In 1917 and 1919 he twice held the rank of acting Major while commanding a divisional signals company, and in 1920 he was acting Lieutenant-Colonel while still a substantive Captain. In 1922 he left the Signals to attend staff college, then in 1924 he was appointed to the War Office as Staff Captain. In 1927 he was finally promoted to Major. In 1934, having been promoted to Lieutenant-Colonel, he was appointed to command his old battalion, the 2nd battalion East Yorkshire Regiment.
In 1939, at the outbreak of World War II, Brigadier Smallwood commanded the Nigerian Brigade. By 1940, his brigade was named the 3rd Nigerian Brigade. In July 1940, under the terms of a war contingency plan, the 3rd Nigerian Brigade was sent to East Africa. There, the Nigerian brigade was joined by two brigades of the King's African Rifles (KAR) to form the 1st African Division. Smallwood was the acting Commanding Officer of this division during its formation.
When Major-General H. E. de R. Wetherall officially took command of the 1st African Division, Smallwood once again reverted to command of his original brigade, now named the 23rd Nigerian Brigade.
In 1942, Smallwood was promoted to Major-General.
- 1937 to 1939 General Staff Officer 1, British Military Mission to Egypt
- 1939 to 1940 Commanding Officer, Nigerian Brigade, West Africa
- 1940 Commanding Officer, 3rd Nigerian Brigade, West Africa
- 1940 Acting General Officer Commanding, 1st African Division, East Africa
- 1940 to 1941 Commanding Officer, 23rd Nigerian Brigade, East Africa
- 1942 General Officer Commanding, Madagascar
- 1945 Head Training Staff, Greek Army
- 1946 Retired
- SMALLWOOD, Maj.-Gen. Gerald Russell, Who Was Who, A & C Black, 1920–2015 (online edition, Oxford University Press, 2014)
- Military profile, generals.dk (incorrect spelling "Gerard")
- The London Gazette: . 7 April 1911.
- The London Gazette: . 23 April 1912.
- The London Gazette: . 3 December 1912.
- The London Gazette: . 26 March 1915.
- The London Gazette: . 14 May 1915.
- The London Gazette: . 28 January 1916.
- The Edinburgh Gazette: . 5 June 1916.
- The London Gazette: . 18 May 1917.
- The London Gazette: . 28 February 1919.
- The London Gazette: . 30 January 1920.
- The London Gazette: . 28 February 1922.
- The London Gazette: . 19 February 1924.
- The London Gazette: . 29 April 1927.
- The Army – The East Yorkishire Regiment, The Times, London, 7 May 1934, page 3
- The London Gazette: . 29 January 1937.
- The London Gazette: . 2 February 1940.