How could they treat a gunshot wound to the stomach?

Surgery!: I am going to assume you mean the organ "stomach" and not just the area of the body "stomach." A gunshot wound to the stomach would be repaired in the operating room with the patient asleep. An incision in the midline of the abdomen would be made to allow for careful examination of all the organs in the abdomen, including the stomach. Injuries would be sewn up and then the abdominal cavity rinsed.

Surgery. : Assuming the bullet entered the abdominal cavity then surgery is necessary. The abdomen must be explored to ensure that no injuries are missed.

Surgery: Gunshot wounds to the abdomen are typically treated with surgery to identify and repair the injuries. Rarely, they can be evaluated and observed without surgery, but that is the exception.