Gross graduated from DeWitt Clinton High School and then attended Harvard College, graduating in 1927. He then attended Oxford University before returning to Cambridge, Massachusetts to attend Harvard Law School.
In 1931, Gross start his career at the United States Department of State as a legal adviser. Two years later, in 1933, he joined the National Recovery Administration, although he stayed for only a year before he moved on to become counsel to the National Association of Manufacturers. In 1940, Gross returned to government service and became an associate counsel of the National Labor Relations Board.
In 1943, during the World War II, Gross was commissioned as a captain in the United States Army. Later, he was promoted to lieutenant colonel. In the Army, Gross served as chief of the economic section of the Civil Affairs Division of the General Staff of the United States Department of War.
After the war, Gross back to the State Department, to the position of the Legal Adviser of the Department of State. He also was a deputy to the Assistant Secretary of State for Occupied Areas, Gen. John H. Hilldring, then, from 1947, Charles E. Saltzman. In 1948, Gross signed the Convention on the Prevention and Punishment of the Crime of Genocide on behalf of the United States. From March through October 1949, Gross was the Assistant Secretary of State for Legislative Affairs.
On October 11, 1949, Gross was appointed the United States' deputy delegate to the United Nations, by Dean Acheson, a United States Secretary of State.
In 1953, Gross joined the law firm of Curtis, Mallet-Prevost, Colt & Mosle; he would remain associated with the firm for the rest of his life. Gross achieved notoriety as a lawyer later his career when in the 1960s he brought a suit in the World Court challenging South Africa's policy of apartheid.
Also in the 1960s Gross assisted the Tibetan government in exile and its leader the Fourteenth Dalai Lama, including advising on preparation of a constitution.
Gross was also active in the international affairs activities of the ecumenical movement. He was a member of the Commission of the Churches on International Affairs of the World Council of Churches (CCIA/WCC) (dates to be inserted) and he chaired the Department of International Affairs of the National Council of Churches of Christ in the USA (DIA/NCCCUSA) from 1954 to 1958. In that capacity he also chaired the NCCCUSA's Fifth World Order Study Conference on the Churches and World Order, in Cleveland, Ohio, 18–21 November 1958.
Gross died on May 2, 1999 at his home in New York City.
On September 1, 1933, Gross married Kathryn Watson. The couple had three children: Suzanne, Peter, Catherine.