Trastuzumab Deruxtecan for Subjects With HER2-Positive Gastric Cancer or Gastro-Esophageal Junction Adenocarcinoma After Progression on or After a Trastuzumab-Containing Regimen (DESTINY-Gastric04)

* Adults (according to local regulation) and able to provide informed consent for study participation.* Pathologically documented gastric and GEJ adenocarcinoma that has been previously treated in the metastatic setting (unresectable, locally advanced, or metastatic disease). * Progression on or after first-line therapy with a trastuzumab or approved trastuzumab biosimilar-containing regimen. Note: Prior neoadjuvant or adjuvant therapy with a trastuzumab-containing regimen can be counted as a line of therapy if the subject progressed on or within 6 months of completing neoadjuvant or adjuvant therapy. Prior neoadjuvant or adjuvant therapy that does not include trastuzumab will not be counted as a line of therapy regardless of the progression status of the subject. * Locally or centrally confirmed HER2-positive (IHC 3+ or IHC 2+ and evidence of HER2 amplification by ISH) as classified by ASCO-CAP on a tumor biopsy obtained after progression on or after a first-line trastuzumab or approved trastuzumab biosimilar-containing regimen. * Eastern Cooperative Oncology Group performance status of 0 or 1 at Screening. * Adequate bone marrow, renal, hepatic function, and blood clotting function within 14 days of randomization.