After the war 1940s, Gerard became involved in his family's real estate business, the Aeon Realty Company, with interests in Manhattan, Long Island, and New Jersey. However, Gerard wished to move to the western United States. He began studying ranching, and in 1947 toured several western states, deciding on Montana. In 1948, he purchased what is now known as the Bar 7 Ranch in Ennis, Montana, moving there with his family in 1949. Gerard operated the ranch as both a home and a livestock operation, raising cattle and horses. He also owned another ranch in the town of Dillon. However, the ranch was never profitable, and Gerard received assistance from his father through monies and stocks in order to help him reduce with his significant debt. Thanks to his father's financial assistance as well as that of a company owned by his family, Gerard was able to maintain a comfortable lifestyle, covering ranch expenses, educational expenses for his children, and costs of operating a private airplane. In addition to ranching, Gerard had business interests in Billings and Bozeman, and was a mining executive for Newmont Resources and a director of Cardinal Petroleum, both based in Billings.
Politics
Sometime after moving to Montana, Gerard became involved in state politics. In 1954, he was elected to the Montana Legislature as a Republican representing Madison County, with his term beginning in 1955. He was elected to three terms in the House, serving in his final term as minority leader from 1959 to 1961. On December 17, 1959, Gerard announced his campaign for the Republican nomination for the United States Senate, and said:
convinced that Montana must look and think ahead, or we will forfeit our potential. I intend to file for nomination to the U.S. Senate with the hope that Montana will send new blood with a fresh outlook to Washington to best represent all Montanans."
However, Gerard lost the nomination in the Republican primary, finishing second in a six-way race, earning 27% of the vote, compared to 39% of votes for nominee Orvin B. Fjare. His father helped fund his campaign, which cost $20,000. One source writes that he may have lost in the primary "because Montanans did not believe him to be authentically Montanan." In 1962, Gerard earned the Republican nomination for the Montana Senate from Madison County. He won the election, serving in the Senate from 1962 to 1966. He was elected minority leader, serving in that capacity from 1965 to 1966.
After leaving his ambassadorship, Gerard relocated to Florida from New Jersey. He was an active benefactor of marine biology and a frequent sponsor of underwater archaeological expeditions and in 1977 became an adjunct professor of maritime archaeology at the University of Miami.
Personal life
Gerard was married twice: first to Louise Taft Grosvenor in 1944, before divorcing in 1966, and next to Teresa Dabrowska, a native of Warsaw, Poland, whom he married in the 1960s and divorced in 2004. He had five children with his first wife: Jenny, Molly, Helen, Anne, and Sumner.
Death
Gerard died of natural causes in a hospital in Vero Beach, Florida, on February 24, 2005. His death was announced by his son, also named Sumner. A memorial service was held at the Smithsonian Marine Station in Fort Pierce, Florida, on March 1, before an afternoon funeral service at St. Thomas Episcopal Church in Manhattan on March 3. The interment was private.