Patients in both treatment groups will be given the same chemotherapy, on an outpatient basis unless special circumstances require hospitalization. Gemcitabine is infused over 30 to 60 minutes through an intravenous catheter (IV)-a thin plastic tube inserted into an arm vein. Herceptin is then given over 30 to 90 minutes through the same IV line. Radiation therapy to the abdomen will start the same day, after the drugs have been administered, and will continue for the next 4 days. This treatment cycle-chemotherapy plus 5 days of radiation therapy-will be repeated each week for 6 weeks.