A 52-year-old Indian woman presents with a 3-month history of progressive abdominal distension and early satiety. On examination, she has a firm, fixed, irregular mass palpable in the epigastrium. Laboratory investigations show hemoglobin 9.2 g/dL, albumin 2.8 g/dL, and elevated CA 19-9 (320 U/mL). Contrast-enhanced CT abdomen reveals a 6 cm heterogeneous mass in the gastric antrum with involvement of the serosa and ascites. What is the most likely diagnosis?