He returned to government service in 1940 when he became associate counsel of the National Labor Relations Board.
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.
In 1948, Gross signed the Convention on the Prevention and Punishment of the Crime of Genocide on behalf of the United States.
The major issue facing the United Nations at that time was the Soviet Union's proposal that, with the conclusion of the Chinese Civil War, the Communist People's Republic of China should replace the Republic of China on the United Nations Security Council.
Malik remained absent for several months, and as such the Soviet Union failed to exercise its veto power to block United Nations Security Council Resolution 82, which condemned North Korea at the beginning of the Korean War; on behalf of the U.S., Gross voted in favor of the resolution.
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.