Paclitaxel and Ramucirumab +/- Zanidatamab in HER2 Postive Advanced Gastroesophageal Adenocarcinoma

Inclusion Criteria: * Participants must have histologically or pathologically confirmed gastroesophageal adenocarcinoma (stomach, gastroesophageal junction or esophagus) that is unresectable or metastatic and which must be HER2 positive as confirmed by central testing using FDA-approved HER2 assay. HER2 positive is defined as IHC 3+, or IHC 2+ and FISH positive. * Participants must have received and failed at least one prior trastuzumab-containing regimen in combination with platinum-based chemotherapy for treatment of locally advanced or metastatic disease. Failure is defined as demonstrated objective disease progression (radiologic) on the most recently administered HER2 targeting agent. * Participants must have presence of measurable or evaluable disease as defined by Response Evaluation Criteria in Solid Tumours (RECIST 1.1). * Participants must be considered a suitable candidate for, and able to receive chemotherapy for advanced disease with paclitaxel and ramucirumab. * Participants must consent to the provision of samples of blood, serum and plasma in order that the specific correlative marker assays may be conducted. * Participants must consent to provision of, and investigator(s) must confirm access to a representative formalin fixed paraffin embedded (FFPE) block of tumour tissue/ or a predetermined number of freshly cut slides of representative tumour tissue of adequate amount and quality in order that the central HER2 testing may be done * Participants must be ≥ …