Additional Study Information: You are being invited to take part in a research study that includes studying DKN-01 combined with another investigational drug called tislelizumab and your physicians choice of a combination of drugs generally used to treat your type of cancer, either capecitabine and oxaliplatin (referred to as CAPOX) OR leucovorin calcium, fluorouracil, and oxaliplatin (referred to as mFOLFOX6) in advanced gastric or gastroesophageal junction (GEJ) cancer. DKN-101 works by targeting the DKK1 protein, which is present in high levels in your type of tumor. The purpose of this study is to find out if combining DKN-01 with tislelizumab and CAPOX (capecitabine + oxaliplatin) or mFOLFOX6 (leucovorin calcium + fluorouracil + oxaliplatin) will be safe and if it will delay the chance of your cancer growing or spreading.