A 36-year-old man presents with weight loss and a large palpable tumor in the upper abdomen. Endoscopy reveals an intact gastric mucosa without signs of carcinoma. Multiple biopsies show normal gastric mucosa. A UGI study shows a mass in the stomach. At surgery, a 3-kg mass is removed. It is necessary to remove the left side of the transverse colon. What is the most likely diagnosis?