On 31 May 1917 he entered the School of Military Aeronautics at the University of Illinois for preliminary flight training, but obtained an honourable discharge on the grounds of dependency on 25 July 1917.

79 Squadron was flying the Sopwith Dolphin, an unusual biplane distinguished by its "negative stagger" wing arrangement, a type Gillet had not flown before.

After becoming acclimated to the Dolphin and honing his combat skills, he scored his first victory on 3 August, destroying a kite balloon north of Estaires.

[7] He returned to the United States and obtained his release from the U.S. Army Signal Reserve Corps and entered the family business.

Gillet pursued a successful, lengthy business career as a liquor importer, realtor, and banker, and died at the age of 74 on 21 December 1969 at the Greater Baltimore Medical Center.