By direction or instinct from God, who both qualified him for and called him to the office of a judge. See Judges 12:7 Hebrews 11:32. Otherwise they might not have chosen a bastard, Deuteronomy 23:2. Unless we will say, that there being no other person among them fit for and willing to this work, necessity dispensed with this law, as it did with other positive laws, as those of the sabbath and sacrifices.
Related Commentaries of Judges 11:5
Judges 11:5
5 And it was so, that when the children of Ammon made war against Israel, the elders of Gilead went to fetch Jephthah out of the land of Tob: