[4] In March 1915, he was awarded the Military Cross (MC): the citation for the medal reads:[5] For gallantry and marked ability at Cuinchy on the 10th January, 1915, when in the forward artillery observation station, and for leaving his trench under fire on two occasions to repair telephone wires.Later in the year fought in the Gallipoli campaign where he was twice mentioned in dispatches and, after this, he spent the rest of the war as a staff officer.

[4] By the end of the war, Gammell was highly decorated; mentioned in dispatches seven times, twice awarded the MC, the first in 1915, and again in 1917, and gained the Distinguished Service Order (DSO) in 1918.

[2][3] The citation for his second MC reads: For conspicuous gallantry and devotion to duty when employed as Staff Officer to the Armoured Car Patrol.

[4] Shortly after the outbreak of the Second World War in September 1939, Gammell, along with his brigade, was deployed to France towards the end of the month, where it became part of the British Expeditionary Force (BEF).

[3] After his brigade spent the first few months of the war digging defensive positions in expectation of a repeat of the trench warfare of 1914–1918, in February 1940 Gammell returned to England where he was given a new role as Chief of Staff of the newly created IV Corps, then commanded by Lieutenant-General Claude Auchinleck, who noted that Gammell "worked like a slave" and that his "energy, determination and devotion to duty were remarkable",[3] and then of Allied Forces Norway during the ill-fated Norwegian Campaign later that year.

Gammell's division was assigned to the GHQ Home Forces reserve, and held in readiness in a counterattack role in the event of a German invasion.

[3] In early January 1944, Gammell was appointed Chief of Staff to the Supreme Allied Commander Mediterranean, General Sir Henry Maitland Wilson.

[15] He was Deputy lieutenant for Angus and, after the death of his first wife, Gertrude (or "G" as he called her), in 1960, he was remarried four years later to Mary Kirkwood, daughter of a Royal Navy officer.