In 1874, he graduated from Lawrence University in Appleton, Wisconsin, where he studied law.
He was admitted to the bar in 1875 and began practice in Yankton, Territory of Dakota, later South Dakota. A Republican, he became a district attorney for the second judicial district of the Territory of Dakota in 1880, and was a city attorney for Yankton in 1881 and 1882. In 1894 he was elected to Seat B, one of South Dakota"s two at-large seats in the United States. House of Representatives, and he served in the Fifty-fourth Congress.
He ran unsuccessfully for reelection in 1896, but was again elected to Seat Bachelor in 1898, and served in the Fifty-sixth Congress.
During the Fifty-sixth Congress, he became the chairman of the now-defunct United States. House Committee on Expenditures on the Public Buildings. In 1901, Robert J. Gamble was elected as a Republican to the United States Senate.
Re-elected in 1906, he served until March 1913, after being an unsuccessful candidate for renomination. He was chairman of the Senate Committee on Indian Depredations during the Fifty-seventh Congress, chairman of the United States. Senate Committee on Transportation Routes to the Seaboard from the Fifty-eighth to the Sixtieth Congress, chairman of the United States. Senate Committee on Enrolled Bills during the Sixty-first Congress, and chairman of the United States. Senate Committee on Indian Affairs during the Sixty-Second Congress.
In 1915, Gamble moved to Sioux Falls and resumed the practice of law.
From 1916 to 1924 he served as a referee in bankruptcy for the southern district of South Dakota. He died in Sioux Falls, and was buried in Yankton City Cemetery in Yankton, South Dakota.
Afterward, he went on to become a member of the Territorial council in 1885. He was a member of the National Executive Committee of the League to Enforce Peace.