This clinical trial is focused on studying a type of cancer called gastric adenocarcinoma, which affects the stomach, and gastroesophageal junction adenocarcinoma, which occurs where the stomach meets the esophagus. The study is investigating the effectiveness of a treatment that combines a medication called pembrolizumab (also known by its code name MK-3475) with chemotherapy, compared to a combination of a placebo and chemotherapy. Pembrolizumab is a type of medication that helps the immune system fight cancer cells. The chemotherapy drugs used in this study include cisplatin, capecitabine, fluorouracil, and oxaliplatin.

The purpose of the study is to compare the overall survival of participants receiving pembrolizumab with chemotherapy to those receiving a placebo with chemotherapy. Participants in the study have HER2-negative cancer, which means their cancer cells do not have a protein called HER2 on their surface. The study is designed to be double-blind, meaning neither the participants nor the researchers know who is receiving the pembrolizumab or the placebo. This helps ensure the results are unbiased.

Participants will receive treatment through intravenous infusion, which means the medication is given directly into a vein, and some medications may be taken orally. The study will follow participants over a period to monitor their response to the treatment and any side effects they may experience. The goal is to determine if pembrolizumab, when combined with chemotherapy, can improve survival rates for people with advanced gastric or gastroesophageal junction adenocarcinoma.