The purpose of this study is to evaluate the safety and efficacy of OBP-301 with pembrolizumab in advanced gastric, gastroesophageal junction, and esophageal adenocarcinoma (gastroesophageal adenocarcinoma).
   
OBP-301 is experimental, which means that it in combination with pembrolizumab is being tested and they are not yet approved by the United States Food and Drug Administration (FDA). 

In this study, an OBP-301 injection will be given every 2 weeks for a total of 4 injection encounters. Participants may receive up to 1 additional tumor injection encounter if the participant has not had disease progression and has had no grade 3 or 4 events related to OBP-301. Participants will also receive pembrolizumab 400 mg IV will be given for 6 weeks until disease progression or for a maximum of 2 years.
   
Participation is expected to last up to 24 months.