Who was Gaal?
Gaal was Ebed's son. He emboldened the Shechemites to rebel against Abimelech (Judg. 9:26). "Gaal came with his brethren and went over to Shechem, and the lords of Shechem (Hebrew) put their confidence in him."
He apparently was captain of a band of freebooters; and the Shechemites who were dissatisfied with Abimelech's rule hoped Gaal would help them against him. Already they had "set liers in wait for Abimelech in the tops of the mountains" (Ebal and Gerizim, between which Shechem was situated), who robbed all passers by.
By organized robbery they brought Abimelech's government into discredit, and probably sought to waylay and kill himself. Gaal developed their brigandage into open revolt. At the vintage ingathering feast they made praise offerings" (hilluliym, KJV made merry, margin songs; compare Isa. 15:9,10) of their fruits, which newly planted vineyards bore in the fourth year, eating and drinking in the house of their god Baal-berith ("Baal in covenant"), answering to Jehovah's feast (Lev. 19:2:3-35).
At the feast Gaal said, "Who is Abimelech and who is Shechem that we should serve him? is not he son of Jerubbaal?" i.e., he is son of the man who pulled down Baal's altar at Shechem and restored Jehovah's worship, for which the Shechemites themselves had tried to slay him (Judg. 6:27-32).
Who is "Zebul his officer"? explains the previous "who is Shechem?" The might of Shechem does not consist in the might of Zebul its prefect, Abimelech's officer. To the one officer of Abimeleeh Gaal opposes, "serve the men of Hamor the father of Shechem " the patricians of the ancient line whom the Shechemites should serve; Humor was the Hivite prince who founded Shechem (Gen. 33:19; 34:2; Josh. 24:32).
The rebellion sought to combine the aboriginal Shechemites with the idolatrous Israelites against the anti-Baalite family of Gideon. Heated with wine Gaal vaunted that he, if made leader of the Shechemites, would soon overcome and "remove Abimelech."
Zebul, jealous of Gaal, privately (literally, with deceit, i.e. feigning assent to Gaal while planning his overthrow) sent information to Abimelech, who (margin, Judg. 9:37) came "by way of the wizards' terebinths," and "chased Gaal" in battle; and "Zebul thrust out him and his brethren that they should not dwell in Shechem."
In Judg. 9:39 it is translated: "Gaal went out in the sight of the lords of Shechem," not at their head, but leading his own men; not until the "morrow" did the Shechemites go out. We know no more of Gaal. Foolhardy boasting, which he failed to make good in action, was his fault.
IBSE, "Isaac" (in the public domain) with minor edits.