Sarah Bradley Gamble, honorary chairman of the Pathfinder Fund of Boston, a foundation that provides money for family planning services and population programs in developing countries, died April 27 at her home in Milton, Mass. She was 86 years old. The fund was established in 1957 by Mrs. Gamble and her husband, the late Dr. Clarence J. Gamble, a leader in birth control research. They traveled extensively in the Middle East and Africa in an attempt to increase the availability of family planning services. Mrs. Gamble was also a former director of the Experiment in International Living, an organization that has sent thousands of Americans abroad and received thousands of foreign youths in the United States. She is survived by two daughters, Sarah G. Epstein of Washingoton and Julia G. Kahrl of Columbus, Ohio; three sons, Dr. Walter J. Gamble of Brookline, Mass., Richard B. Gamble of Cambridge, Mass., and the Rev. Robert D. Gamble, of Milton, Mass.; a sister, Mary B. Emery of Lincoln, Mass., and 17 grandchildren.