You are being asked to take part in Part 2 of this study because you have advanced gastric cancer (a disease in which cancer cells form in the lining of the stomach) or gastroesophageal junction cancer (a type of cancer forming in the area where the tube that connects your mouth and the stomach join together) and have not received prior treatment for your advanced cancer. These cancer types often have an excessive amounts of the fibroblast growth factor receptor 2b (FGFR2b) proteins (a protein possibly causing cells to grow more quickly and multiply abnormally), confirmed during the prescreening test.