This is a phase 1/2 multicenter, open-label umbrella platform study that will evaluate the safety and efficacy of sacituzumab tirumotecan (MK-2870) plus paclitaxel versus ramucirumab plus paclitaxel, and HER3-DXD plus ramucirumab versus ramucirumab plus paclitaxel for the treatment of participants with advanced or metastatic gastric adenocarcinoma, gastroesophageal junction (GEJ) adenocarcinoma, or esophageal adenocarcinoma who have failed 1 prior line of therapy. This is an estimation study, and no formal hypothesis testing will be performed.