The purpose of the study is to test 18F-BMS-986229 positron emission tomography (PET) imaging a practical and safe way to check the status of esophageal, stomach, and gastroesophageal junction cancer.

Inclusion Criteria:

• Participant must have pathologically or cytolofically confirmed (at MSK) esophageal, gastric, or gastroesophageal junction adenocarcinoma or squamous cell carcinoma
• PD-L1-positive expression in >/=1% of the tumor or stromal cells. PD-L1 expression will be reviewed by MSK pathologist, scored and recorded using the combined positive score (CPS) formula (number of PD-L1 positive tumor cells, lymphocytes, and macrophages divided by the number of tumor cells x 100).
• Participants must have disease that can be evaluated radiographically. This may be measurable disease or nonmeasurable disease according to RECIST 1.1.
• Concurrent therapy is allowed.
• Age 18 years or older.
• Eastern Cooperative Oncology Group (ECOG) performance status 0-2.
• Female subjects of childbearing potential must have a negative urine or serum pregnancy test within 24 hours before receiving the first dose of the study medication. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required.
• Adequate organ function, as defined in Table 1. All screening labs should be performed within 14 days of initiation of treatment.

Hematological Absolute neutrophil count: >/=1,000/mCL Platelets: >/=90,000/mCL Hemoglobin: >/=8 g/dL

Renal Serum creatinine: \

Potential research subjects will be identified by members of the patient's treatment team, the protocol investigator or research team. If the investigator is a member of the treatment team, s/he will screen their patient's medical records for suitable research study participants. The investigator will use information provided by the patient and/or medical record to confirm that the patient is eligible and contact the patient regarding study enrollment. All eligible patients, regardless of sex and race will be approached for participation. No additional measures, e.g. advertisement, payment to patients, will be employed to recruit patients.

Memorial Sloan Kettering Cancer Center supports the international committee of medical journal editors (ICMJE) and the ethical obligation of responsible sharing of data from clinical trials. The protocol summary, a statistical summary, and informed consent form will be made available on clinicaltrials.gov when required as a condition of Federal awards, other agreements supporting the research and/or as otherwise required. Requests for deidentified individual participant data can be made beginning 12 months after publication and for up to 36 months post publication. Deidentified individual participant data reported in the manuscript will be shared under the terms of a Data Use Agreement and may only be used for approved proposals. Requests may be made to: crdatashare@mskcc.org.