Then the children of Ammon were gathered together, and encamped in Gilead. The children of Israel assembled themselves together, and encamped in Mizpah.
"He shall be head over all the inhabitants of Gilead"  (Judges 10:18). This means that he would be head over those tribes of Israel which dwelt east of the Jordan River.  Gad, Reuben and half of Manasseh were those tribes.