Gabriel was the elder of the two sons of Farkas Bethlen and Druzsiána Lázár.
According Gabriel's first extant letter (from 1593), Sigismund Báthory, Prince of Transylvania, seized the brothers' estates "at the word of many coaxing people" without paying a compensation to them in 1591 or 1592, but a "few primary kinsmen" convinced the prince to offer restitution or other landed property to them.