‘ And so it happened on the next day, that the people went out into the field, and they told Abimelech.'
The next day some of the people who were with Abimelech went from Arumah into the countryside, probably to survey the situation, and returned to tell him of the expulsion of Gaal and his brothers, something they may have learned from a messenger sent by Zebul. This interpretation is supported by the re-mention of ‘the people' in Judges 9:43.
Alternately it may be that some of the people in the city, thinking that Abimelech had withdrawn, themselves went out to their fields to prepare them for the next stage of ploughing, before Abimelech could attack again. Whatever else happened the supply of food had to be maintained. (This may have been Abimelech's hope when he withdrew). Then this news reached Abimelech, either from scouts or by means of a messenger from the city, from faithful Zebul.