During World War I he was awarded the Purple Heart for bravery in the fighting in the Gérardmer sector of the Western Front and commanded the 318th Engineer Regiment in the Meuse-Argonne Offensive.

He wanted to enter the United States Military Academy at West Point, New York, but was unable to secure an appointment.

[4] His classmates included Brehon B. Somervell, who was ranked sixth,[5] and became a close personal friend;[6] Harry Ingles, who was 36th;[7] and Carl Spaatz, who was 57th.

They had five children: Lucy Helen, Dorothea Katherine, John Edward, Nancy Ellen and Sheldon Harley.

[3] He was promoted to first lieutenant on 28 February 1915, and from 28 September 1915 to 22 December 1916 he attended the Army Engineer School at Washington Barracks.

He was promoted to captain on 15 May 1917, a few weeks after the American entry into World War I, and major on 5 August, commanding a company and then a battalion of the 4th Engineers.

He was promoted again, this time to lieutenant colonel, on 23 August 1918,[4] and was awarded the Purple Heart for bravery in the fighting in the Gérardmer sector.

[4] On 3 October 1919, Gross, who was at the time in charge of fortification work on the Harbor Defenses of Long Island Sound, reverted to his substantive rank of captain, but he was promoted to major again on 1 July 1920.

[15] Gross accompanied W. Averell Harriman and Lord Beaverbrook on the Harriman-Beaverbrook Mission to London and Moscow in September 1941, where he studied the transportation needs of the Red Army.

[3][16] In a sweeping reorganisation of the War Department in March 1942, soon after the United States entered World War II, Gross's Transportation Division of the WDGS was merged with the Transportation Division of the Quartermaster Corps, which controlled field installations including the ports of embarkation and the regulating and reconsignment points that handled the flow of men and supplies.

[14] As the Chief of Transportation, Gross attended the top-level wartime conferences in Quebec in 1943, and Malta, Yalta and Potsdam in 1945.

He rejected collective bargaining with the Transport Workers Union of America, which put him at odds with La Guardia's successor, William O'Dwyer, who sought to improve labor relations.

[16][21] On 15 June 1948, during the Berlin Blockade, Gross returned to active duty at his substantive rank of colonel as the Deputy Chief of the Transport Group in Office of Military Government, United States, in Germany.