Albert Barnes Commentary Judges 9:26

"And Gaal the son of Ebed came with his brethren, and went over to Shechem; and the men of Shechem put their trust in him." — Judges 9:26 (ASV)

It does not appear who Gaal, son of Ebed, was; he may have been an officer sent by Abimelech with a force to bring the men of Shechem back to their allegiance, but who tried to turn the rebellion to his own advantage. He got into Shechem with a band of men, “his brethren,” unopposed by Zebul, Abimelech’s officer, and soon gained the confidence of the Shechemites.