Gillet attended the University of Virginia before joining the United States Air Service on April 1, 1917.  By the end of the year, he transferred to the Royal Flying Corps, receiving his training in Canada and England. On March 29, 1918, he was assigned to 79 Squadron in France. Flying a Sopwith Dolphin, Gillet scored 20 victories by the war's end. In addition to three drachen, he was credited with downing fourteen Fokker D.VIIs.

Distinguished Flying Cross (DFC)
"When attacking a kite balloon, a two-seater guarding it advanced to engage him; Lt. Gillet shot the machine down and turning to the balloon, which was being rapidly hauled down, he dropped two bombs at the winch and fired a drum into the balloon, which deflated but did not catch fire. In addition to this two-seater, this officer has accounted for two other machines and a kite balloon." DFC citation, London Gazette, November 2, 1918

Distinguished Flying Cross (DFC) Bar
"A pilot of great dash and skill who, since August 3, has destroyed twelve hostile aircraft. On September 29, when on low line patrol, he attacked three Fokkers, driving down one, which fell in flames." DFC Bar citation, London Gazette, November 8, 1919