Colonel Cortez will explain to you that Gonzales, his right hand man, took your money. He'll give you a chainsaw and tell you to kill for retribution. Drive to the dot on your map and park your car near the open doors of the building. Tommy will go in the building and a cutscene begins. Gonzales tries to escape and Tommy must chase Gonzales out of the building. (There will be a hidden package near where you start on the building.)
Once outside, get in a car and run him over (you can use the chainsaw provided or any method of killing but it's far easier to run him over, and you need a car as soon as you kill him anyway.) Afterwards, go to the Pay 'n Spray to the North to lose the wanted rating and complete the mission.