Abimelech lived at Arumah: and Zebul drove out Gaal and his brothers, that they should not dwell in Shechem.
The mention in  Judges 9:41 that Gaal and his brethren were forbidden to live in Shechem is probably an indication that they were not involved in the slaughter of the whole city mentioned in the same paragraph.  It appears likely that if Gaal indeed had been killed, that some mention of it would have been found here.