Pembrolizumab in Treating Patients With Metastatic or Unresectable Gastroesophageal Adenocarcinoma

Inclusion Criteria: * The subject (or legally acceptable representative if applicable) provides written informed consent for the trial. The subject may also provide consent for future biomedical research. However the subject may participate in the main trial without participating in future biomedical research. * Be willing and able to provide written informed consent/assent for the trial. * Histologically and cytologically documented diagnosis as gastroesophageal adenocarcinoma. * Have a documented advanced (metastatic and/or unresectable) gastroesophageal adenocarcinoma that is incurable and for which prior first-line or later-line standard of care (SOC) treatments have failed. There is no limit to the number of prior treatment regimens. Prior neoadjuvant or adjuvant therapy included in initial treatment may not be considered first- or later-line SOC treatment unless such treatments were completed less than 12 months prior to the current tumor recurrence. * Have submitted an evaluable tissue sample for biomarker analysis from a newly obtained endoscopic, core, incisional, or excisional biopsy of a tumor lesion not previously irradiated. The tumor tissue submitted for analysis must be from a single tumor tissue specimen and of sufficient quantity and quality to allow biomarker study. A newly obtained tumor specimen, defined as a specimen obtained up to 6 weeks (42 days) prior to initiation of treatment on day 1, for biomarker characterization will be required for enrollment …