After completing flying training, Gardner was assigned to No. 19 Squadron RFC in January 1918, flying the Sopwith Dolphin. On 1 April 1918, the Royal Flying Corps and the Royal Naval Air Service merged to create the Royal Air Force. Gardner's first victory came on 6 June 1918 during the German spring offensive, that had begun in March, pushing the British back over all of the ground they had gained since the Battle of the Somme in 1916, bringing down a DFW C.V over Vieux-Berquin. It was shared with three other pilots, Gordon Budd Irving, Finlay McQuistan, and John De Pencier. Three days later, on 9 June, he destroyed another DFW C.V solo over Neuf-Berquin. In July, he brought down a further four enemy aircraft. 8 August saw the start of the decisive Allied counter-offensive that, in the 100 days up to 11 November 1918, crushed the German Army, and on 11 August Gardner shot down two enemy aircraft in one afternoon. The next day, 12 August, he was appointed a temporary captain. He shared his ninth victory, on 16 September 1918, with James Hardman. His final fight took place on 27 September, when he destroyed a Fokker D.VII over Haynecourt, bringing his total to six enemy aircraft destroyed and four driven down out of control. Gardner was then shot down by German ace "Leutnant" Gustav Borm of Royal Prussian "Jagdstaffel 1", and fatally injured. He died of his wounds three days later.