Gordon Apps followed his elder brother into the Artists Rifles in 1917; he transferred to the Royal Flying Corps in late August. His training took him successively through No. 2 Officer Cadet Wing and No. 2 School of Instruction at Oxford before he transferred to No. 9 Training Squadron at Norwich for flight training on Maurice Farman Shorthorns and Longhorns. After soloing, he shipped out to RAF Turnberry's Gunnery School, then to RAF Ayr's Fighting School. He then went on to Shawbury to polish his skills in both 67 and 10 Training Squadrons. On 22 November 1917, he was forwarded to Italy to join 66 Squadron, arriving 29 December 1917.[1]
On 11 March 1918, he was on patrol at 14,000 feet (4,300 m) altitude with Alan Jerrard and Peter Carpenter when they attacked half a dozen enemy aeroplanes. Apps sent one down out of control; Carpenter saw it drop into a deep gully. Later that month, on the 28th, Apps fired 300 rounds of machine gun ammo into an enemy Albatros D.III, chasing it downwards from 10,000 feet (3,000 m) to just 400 feet (120 m); he was credited with its destruction over Spresiano.[1][2]
On 4 May 1918, Apps set one opposing Albatros down in flames on the banks of the Piave River and downed another making headon firing passes on him. The latter foe crawled from the wreckage only to be strafed by another British pilot; this victim was probably Austro-Hungarian ace Andreas Dombrowski. On 24 May, Apps became an ace while flying as wingman to famed ace "Billy" Barker, being credited with another Albatros D.III destroyed.[1][2]
Apps scored in both morning and evening of 21 June 1918, destroying an Albatros D.V and driving another down out of control. A week later, he destroyed an Albatros D.III. On 13 July, he destroyed an Austro-Hungarian Berg D.I fighter. On the 16th, he shared in driving down an enemy two-seater reconnaissance craft out of control for his tenth win.[1][2]
The day after his last win, on 17 July 1918, Apps was wounded by anti-aircraft shrapnel. On 2 August, he was shipped to France for treatment at 62 General Hospital. When discharged as fit for duty on 5 September, he returned to duty with his home squadron. He spent a short spell with them, during which he claimed an unconfirmed win on 16 September.[1]
He won a Distinguished Flying Cross for his exploits; it was promulgated in the London Gazette 21 September 1918:
A bold and skilful airman who in recent operations has destroyed six enemy aeroplanes, accounting for two in one flight. He displays marked determination and devotion to duty.[3]
Apps returned to Home Establishment in England. His posting over the next month is uncertain, but he spent November and December 1918 in a wireless course at Penshurst. He then served with 50 Squadron until discharged on 19 March 1919.[1]