‘ And when Zebul, the prince of the city, heard the words of Gaal the son of Ebed, his anger was kindled.'
Zebul, Abimelech's deputy, learned of what Gaal had said and grew angry. If he was in the city representing Abimelech we would expect him to be. What it means is that he heard what was being said, possibly through a spy, and reacted accordingly.