Gammage served in the Texas House of Representatives from 1971 to 1973. Gammage was a member of the so-called "Dirty 30," a bipartisan group of legislators that pushed for reform in the 1970s in the wake of the Sharpstown scandal in which then state House Speaker Gus Mutscher of Brenham in Washington County was convicted and sentenced to five years probation for conspiring to accept a bribe. As a legislator he advocated government reform, consumer and health legislation, voting rights for eighteen-year -olds, and equal rights for women. He was a member of the Texas State Senate from 1973 to 1976, when he was elected to the 95th Congress, having unseated then freshman Republican Ron Paul. He served only one term in Congress, having been unseated by Paul in 1978. From 1979 to 1980, Gammage was assistant state attorney general under Attorney General Mark Wells White. In 1980, he was a special consultant to the U.S. Department of Energy under U.S. President Jimmy Carter, the last Democrat to win the electoral votes of Texas.
In 1982, Gammage was elected as a justice to the Texas Third Court of Appeals in Austin and served in that position until 1991. He was elected in 1990 to the Texas Supreme Court, on which he served from 1991 until 1995. During his time on the bench Gammage participated in nearly 250 cases. He embraced an expansive interpretation of the legal doctrines and constitutional provisions that protect individual rights and equality. Gammage garnered national attention when he resigned from the Texas Supreme Court in 1995 to draw attention to the increasing amount of influence that campaign contributors and political action committees (PACs) had on judicial elections. Working with other proponents of judicial reform, including former Texas State Supreme Court Chief Justice Thomas R. Phillips, Gammage was a key actor in bringing about caps on campaign contributions in judicial elections.
In the late 1990s and early 2000s (decade), he taught at Sam Houston State University, Texas A&M University-Corpus Christi, Texas State University in San Marcos, and Roman Catholic-affiliated St. Edwards University in Austin. In 2006, Gammage lost the Texas gubernatorial Democratic primary election to former U.S. Representative Chris Bell of Houston. Bell was then defeated by incumbent Republican Rick Perry. On May 27, 2008, Gammage delivered the funeral eulogy for his former "Dirty Thirty" colleague Joseph Hugh Allen, a former representative from Baytown. Later in 2008, Gammage worked in the unsuccessful campaign to nominate Hillary Clinton for U.S. president, having traveled to Iowa to meet with voters. According to his wife, Lynda, he spent his last years often performing pro bono legal work for the needy. Gammage died at the age of 74 in his Llano home of an apparent heart attack on September 10, 2012.
30° 15.934, -097° 43.639
Texas State Cemetery