‘ And he sent messengers to Abimelech secretly, saying, “Gaal the son of Ebal, and his brothers have come to Shechem, and behold they are constraining the city against you.” '
Gaal and his brothers were probably well known as troublemakers, possibly as Habiru, always seeking to stir up trouble and obtain rich pickings for themselves. So Zebul let Abimelech know that they were there and what they were doing. The word translated ‘constrain' usually means ‘besiege' but here it is used metaphorically to describe the besieging of the mind.