The purpose of this observational study is to assess the effectiveness and safety of nivolumab in participants with previously untreated advanced or metastatic Gastric Cancer (GC), Gastro-oesophageal Junction (GEJ) Adenocarcinoma, or Oesophageal Adenocarcinoma (EAC).

Inclusion Criteria:

• Diagnosis of HER2 negative advanced or metastatic HER2-negative GC, GEJ adenocarcinoma or EAC whose tumours express PD-L1 with a CPS ≥ 5
• Whose physician has decided to start a treatment with nivolumab (according to the German marketing authorization) for the treatment of GC, GEJ adenocarcinoma or EAC and prior to study participation

Exclusion Criteria:

• Previous malignancy within 3 years or concomitant malignancy, except: those with a 5-year overall survival rate of more than 90%, e.g. non-melanomatous skin cancer or adequately treated in situ cervical cancer.
• Participants currently included in an interventional clinical trial for their advanced or metastatic GC, GEJ adenocarcinoma or EAC. Participants who have completed their participation in an interventional trial or who are not receiving any study drug anymore and who are only in the follow-up Phase for OS can be enrolled. For blinded studies, the study drug administered needs to be known at the time of enrolment.

This study will include participants with previously untreated advanced or metastatic GC, GEJ adenocarcinoma or EAC that will be enrolled continuously.