A Study of PET Scans With the Radioactive Tracer 18F-BMS-986229 in Patients With Esophageal, Stomach, or 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: \</=2.0 x upper limit of normal (ULN) Hepatic Serum total bilirubin: \</=2.5 x ULN (1.5 mg/dL or 25.65 umol/L) OR (except patients with Gilbert's disease \[\</=3 x ULN\]) AST (SGOT) and ALT (SGPT): \</=2.5 x ULN OR \</=5 x ULN for subjects with liver metastases Exclusion Criteria: * Known diagnosis of immunodeficiency or receipt of systemic steroid therapy or any other form of immunosuppressive therapy within 7 days before the first dose of trial treatment * History of active tuberculosis (Bacillus tuberculosis) * Active or documented autoimmune or inflammatory disorder (including inflammatory bowel disease, systemic lupus erythematous, Wegener syndrome \[granulomatosis with polyangitis\], myasthenia gravis, Graves' disease, rheumatoid arthritis, hypophysitis, uveitis) within the 3 years before the start of treatment. The following are exceptions to this criterion: * Subjects with vitiligo or alopecia * Subjects with hypothyroidism (e.g., following Hashimoto syndrome) who are stable with hormone replacement therapy or with psoriasis not requiring systemic treatment