Inclusion Criteria:

          1. Signed and dated informed consent,

          2. Age ≥18 years to ≤75 years of age,

          3. Histologically proven non-metastatic gastric adenocarcinoma or of the oeso-gastric
             junction T2 to T4, Nx, M0 after thoraco-abdominopelvic computed tomography (CT) and
             echo-endoscopy,

          4. Subjects must be willing and able to comply with scheduled visits, treatment schedule,
             laboratory tests, tumor biopsies, and other requirements of the study,

          5. dMMR (protein expression by immunohistochemistry [ICH] and/or MSI by polymerase chain
             reaction [PCR]), MMR and/or MSI tumors should be assessed per local guidelines: ICH
             with two (anti-MLH1 and anti-MSH2 or anti-MSH6, and antiPMS2) or four antibodies
             (anti-MLH1, anti-MSH2, anti-MSH6, and anti-PMS2) and/or PCR (with PROMEGA: BAT- 25,
             BAT-26, NR-21, NR-24, and NR-27) by the investigators prior to screening, Extinct MLH1
             (+/- PMS2), MSH2 (+/- MSH6), MSH6, or PMS2 alone protein expression by IHC (dMMR),
             and/or tumor with ≥ 2 instable MSI-H markers on PCR: BAT25, BAT26, NR21, NR24, and
             NR27 (pentaplex panel is recommended),

          6. The Eastern Cooperative Oncology Group Performance status (ECOG PS) 0-1, for patients
             over 70 years ECOG PS of 0;

          7. Hematological status: absolute neutrophil count (ANC) ≥1.5 x 109/L; platelets ≥100 x
             109/L; hemoglobin ≥9 g/dL,

          8. Adequate renal function: serum creatinine level <120 µM, clearance > 50ml/min
             (Modification of the Diet in Renal Disease [MDRD] or Cockcroft and Gault),

          9. Adequate liver function: serum bilirubin ≤1.5 x upper normal limit (ULN), alkaline
             phosphatase <5 x ULN, alanine aminotransferase (ALT), and aspartate aminotransferase
             (AST) ≤3.0 x ULN,

         10. No prior therapy for localized oeso-gastric cancer,

         11. Radiological tumor assessment within 21 days before the start of treatment according
             to RECIST version 1.1 by Chest Abdomen and Pelvis CT,

         12. For female patients of childbearing potential, negative pregnancy test within 7 days
             before starting the study drug,

         13. Men and women are required to use adequate birth control during the study (when
             applicable), Female participants of childbearing potential and male participants with
             partners of childbearing potential must agree to use a highly effective method of
             birth control (i.e., pregnancy rate of less than 1% per year) during the period of
             treatment and during 5 and 7 months, woman and men, respectively, from the last
             treatment administration. Men must refrain from donating sperm during this same
             period, Contraceptive methods that result in a low failure rate when used consistently
             and correctly include methods such as combined hormonal contraception associated with
             inhibition of ovulation (oral, intravaginal, transdermal), progestogen-only hormonal
             contraception associated with inhibition of ovulation (oral, injectable, implantable),
             some intrauterine devices, intrauterine hormone- releasing stem, true sexual
             abstinence (when this is in line with the preferred and usual lifestyle of the
             participant), bilateral tubal occlusion, or a female partner who is not of
             childbearing potential or a male partner who has had a vasectomy. Women and female
             partners using hormonal contraceptive must also use a barrier method i.e. condom or
             occlusive cap (diaphragm or cervical/vault caps), A woman is considered to be of
             childbearing potential if she is postmenarcheal, has not reached a postmenopausal
             state (>12 continuous months of amenorrhea with no identified cause other than
             menopause), and has not undergone surgical sterilization (removal of ovaries and/or
             uterus),

         14. Subject willing to comply to provide primary tumor tissue (archival or fresh biopsy
             specimen), including possible pre-treatment biopsy for PD-L1 expression analysis and
             other biomarker correlative studies

         15. Registration in a National Health Care System (PUMa - Protection Universelle Maladie
             included)

        Exclusion Criteria:

        Non-eligible to clinical trial if one of following parameter is reported:

          1. Concomitant unplanned antitumor therapy (e.g. chemotherapy, molecular targeted
             therapy, immunotherapy),

          2. Treatment with any investigational medicinal product within 28 days prior to study
             entry,

          3. Major surgical procedure within 4 weeks prior to initiation of study treatment,

          4. Other serious and uncontrolled non-malignant disease (including active infection),

          5. Other concomitant or previous malignancy, except: i/ adequately treated in-situ
             carcinoma of the uterine cervix, ii/ basal or squamous cell carcinoma of the skin,
             iii/ cancer in complete remission for >5 years,

          6. Metastases (M stage disease) whatever the location,

          7. Pregnant or breastfeeding women,

          8. Human immunodeficiency virus (HIV),

          9. Active hepatitis B virus (HBV, defined as having a positive hepatitis B surface
             antigen [HBsAg] test prior to inclusion) or hepatitis C virus (HCV). Note: Patients
             with past HBV infection or resolved HBV infection (defined as having a negative HBsAg
             test and a positive antibody to hepatitis B core antigen antibody test) are eligible.
             Note: Patients positive for HCV antibody are eligible only if PCR testing is negative
             for HCV RNA

         10. Patient on tutelage or guardianship or under the protection of justice.

         11. Impossibility of submitting to the medical follow-up of the study for geographical,
             social or psychiatric reasons.

        Non-eligible to immunotherapy:

          1. History of autoimmune disease including, but not limited to myasthenia gravis,
             myositis, autoimmune hepatitis, systemic lupus erythematosus, rheumatoid arthritis,
             inflammatory bowel disease, vascular thrombosis associated with antiphospholipid
             syndrome, Wegener's granulomatosis, Sjögren's syndrome, Guillain-Barré syndrome,
             multiple sclerosis, vasculitis, or glomerulonephritis, Note: History of
             autoimmune-related hypothyroidism on a stable dose of thyroid replacement hormone may
             be eligible. Note: Controlled Type 1 diabetes mellitus on a stable insulin regimen may
             be eligible.

          2. History of idiopathic pulmonary fibrosis (including pneumonitis), drug-induced
             pneumonitis, organizing pneumonia (i.e., bronchiolitis obliterans, cryptogenic
             organizing pneumonia), or evidence of active pneumonitis on screening chest imaging,

          3. Administration of a live, attenuated vaccine within 4 weeks prior to start of
             treatment or anticipation that such a live attenuated vaccine will be required during
             the remainder of the study,

          4. Prior treatment with CD137 agonists, anti-CTLA4, anti-PD-1, or antiPD-L1 therapeutic
             antibody or pathway-targeting agents,

          5. Prior allogeneic bone marrow transplantation or prior solid organ transplantation,

          6. Treatment with systemic corticosteroids or other systemic immunosuppressive
             medications (including, but not limited to prednisone, dexamethasone,
             cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-tumor necrosis
             factor agents) within 2 weeks prior to start of adjuvant treatment, or requirement for
             systemic immunosuppressive medications during the remainder of the study. Inhaled or
             topical steroids and adrenal replacement doses >10 mg daily prednisone equivalents are
             permitted in the absence of active autoimmune disease.

        Note: Patients who have received acute, low-dose, systemic immunosuppressant medications
        (e.g., a one-time dose of dexamethasone for nausea) may be enrolled into the study after
        approval of the Medical Contact. Subjects are permitted the use of topical, ocular, intra-
        articular, intranasal, and inhalational corticosteroids (with minimal systemic absorption).

        Adrenal replacement steroid doses including doses >10 mg daily prednisone is permitted. A
        brief (less than 3 weeks) course of corticosteroids for prophylaxis (e.g. contrast dye
        allergy) or for treatment of non-autoimmune conditions (e.g. delayed-type hypersensitivity
        reaction caused by a contact allergen) is permitted