As the son of a harlot, Jephthah has no legal standing in the tribe. Gilead begat] Throughout the rest of the narrative Gilead is the name of a place, not a person (cp. Judges 12:7). Here Gilead's 'sons' represent the legitimate tribesmen.
Related Commentaries of Judges 11:1
Judges 11:1
1 Now Jephthaha the Gileadite was a mighty man of valour, and he was the son of an harlot: and Gilead begat Jephthah.