Born in Hendricks County, Indiana, on October 29, 1914, Gayle was the second child of Harry C. Thornbrough, an engineer, and Bess Tyler, his wife.
She also traveled extensively with her older sister, Emma Lou, who was an Indiana historian, author and professor of history at Butler University.
After Indiana archaeologist Glenn Albert Black died in 1964, leaving behind an uncompleted manuscript of his work at Angel Mounds, Thornbrough commissioned others to finish Angel Site: An Archaeological, Historical, and Ethnological Study (Indianapolis: Indiana Historical Society, 1967) and edited the final manuscript to assure continuity of style.
[4] Thornbrough left the IHS staff in 1967, when she went to work for the Library of Congress in Washington, D.C. She spent twenty months as a specialist in early U.S. history in its Manuscript Division.
Fletcher, a prominent Indianapolis resident, began his diary in 1820 and continued the effort with some periodic gaps for nearly twenty years.
[7] Thornbrough was appointed the IHS's first executive secretary in 1976, following its reorganization as a separate entity from the Bureau, a state-supported historical agency.
[8] The Indiana Magazine of History's annual Thornbrough award, a tribute to Gayle, her sister, Emma Lou, and their contributions to the historical profession, recognizes the best article to appear in its pages.