Gamaliel Bradford was born on October 9, 1863 in Boston, Massachusetts, United States. He was a son of Gamaliel Bradford V, a banker, and Clara Crowninshield (Kinsman) Bradford.
In 1965, the family moved to Wellesley, Massachusetts where soon his mother died. Gamaliel lived at 493 Worcester Street of this town till the end of his life.
Gamaliel Bradford was the sixth of this name descendents of Plymouth Colony Governor William Bradford. His grandfather, Dr. Gamaliel Bradford IV, was an American physician and the adherent of abolitionism.
Bradford was a sickly child and remained a semi-invalid all his life, suffering from various health problems including severe vertigo, sciatica, rheumatism, and a number of intestinal disorders.