“ And it shall be, that, in the morning, as soon as the sun is up, you will rise early, and set upon the city, and, behold, when he and the people who are with him come out against you, then you may do to them as your hand will find.”
Then when morning came they were to attack the city, and when Gaal and his supporters came out to meet them they could do to them whatever was necessary. He had outlined the primary strategy. At that point it would be up to Abimelech.