Study of Atezolizumab + FLOT vs. FLOT Alone in Patients With GC/GEJ and High Immune Responsiveness

Inclusion Criteria: 1. Have provided written informed consent 2. In the investigator's judgement, is willing and able to comply with the study protocol including the planned surgical treatment 3. Female and male patients\* ≥ 18 years of age 4. Diagnosed with histologically confirmed adenocarcinoma of the GEJ (Type I-III) or the stomach (cT2, cT3, cT4, any N category, M0), or (any T, N+, M0) that: 1. is not infiltrating any adjacent organs or structures by CT or MRI evaluation 2. does not involve peritoneal carcinomatosis 3. is considered medically and technically resectable Note: the absence of distant metastases must be confirmed by CT or MRI of the thorax and abdomen, and, if there is clinical suspicion of osseous lesions, a bone scan. If peritoneal carcinomatosis is suspected clinically, its absence must be confirmed by laparoscopy. Diagnostic laparoscopy is mandatory in patients with T3 or T4 tumors of the diffuse type histology in the stomach. 5. No prior cytotoxic or targeted therapy 6. No prior partial or complete esophagogastric tumor resection 7. ECOG ≤ 1 8. Phase II only: Availability of a representative tumor specimen that is suitable for determination of PD-L1 and MSI status; MSI assessment will be performed locally or centrally, and result must be available prior to randomization (for details, see chapter 9.1). PD-L1 will be assessed centrally but is not used for enrolment of the patients. The analysis requires paraffin embedded biopsy samples of the tumor. Phase III only: Assessment of MSI and PD-L1 \[and optional TMB/EBV\] must be performed locally and results for either of the following MSI-high, PD-L1 CPS≥1, TMB ≥10/MB or EBV+ must be available prior to randomization (for details, see chapter 9.2). 9. Females of childbearing potential must agree to remain abstinent (refrain from heterosexual intercourse) or use contraceptive methods that result in a failure rate of \<1% per year during the treatment period and for at least 5 months after the last study treatment. A woman is considered to be of childbearing potential if she is postmenarcheal, has not reached a postmenopausal state (has not had ≥12 continuous months of amenorrhea with no identified cause other than menopause), and has not undergone surgical sterilization (removal of ovaries and/or uterus). Examples of contraceptive methods with a failure rate of \< 1% per year include bilateral tubal ligation, male sterilization, hormonal implants, established, proper use of combined oral or injected hormonal contraceptives, and certain intrauterine devices. The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception. Males must agree to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures, and agree to refrain from donating sperm, as defined below: a. With female partners of childbearing potential or pregnant female partners, men must remain abstinent or use a condom plus an additional contraceptive method that together result in a failure rate of 1% per year during the treatment period and for at least 3 months after the last dose of study treatment to avoid exposing the embryo. Men must refrain from donating sperm during this same period. Men with a pregnant partner must agree to remain abstinent or to use a condom for the duration of the pregnancy. 10. Criterion integrated in criterion 9. 11. Adequate hematological, hepatic and renal function as indicated by the following parameters: * Leukocytes ≥ 3.000/mm³, platelets ≥ 100.000/mm3without transfusion, absolute neutrophil count (ANC) ≥ 1500/mm3without granulocyte colony-stimulating factor support, Hemoglobin ≥ 90 g/L (9 g/dL) - Patients may be transfused to meet this criterion. * Bilirubin ≤ 1.5 x upper limit of normal, aspartate transaminase and alanine transaminase ≤ 2.5 x upper limit of normal, alkaline phosphatase ≤ 2.5 x upper limit of normal * Serum creatinine ≤ 1.5 x upper limit of normal, or glomerular filtration rate \> 45 ml/min (calculated using the Cockcroft-Gault formula) * Serum albumin ≥ 25 g/L (2.5 g/dL) * For patients not receiving therapeutic anticoagulation: INR or aPTT ≤ 1.5 x ULN; for patients receiving therapeutic anticoagulation: stable anticoagulant regimen \*There are no data that indicate special gender distribution. Therefore, patients will be enrolled in the study gender-independently. Exclusion Criteria: 1. History of severe allergic, anaphylactic, or other hypersensitivity reactions to chimeric or humanized antibodies or fusion protein; Known hypersensitivity to Chinese hamster ovary cell products or to any component of the atezolizumab formulation 2. Any known contraindication (including hypersensitivity) to docetaxel, 5-FU, leucovorin, or oxaliplatin. 3. Active or History of autoimmune disease including, but not limited to, myasthenia gravis, myositis, autoimmune hepatitis, systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease, antiphospholipid antibody 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, or controlled Type 1 diabetes mellitus on a stable insulin regimen may be eligible based on consultation with the sponsor's medical monitor. Patients with eczema, psoriasis, lichen simplex chronicus, or vitiligo with dermatologic manifestations only (e.g., patients with psoriatic arthritis are excluded) are eligible for the study provided all of following conditions are met: * Rash must cover \< 10% of body surface area * Disease is well controlled at baseline and requires only low-potency topical corticosteroids * No occurrence of acute exacerbations of the underlying condition requiring psoralen plus ultraviolet A radiation, methotrexate, retinoids, biologic agents, oral calcineurin inhibitors, or high potency or oral corticosteroids within the previous 12 months 4. Prior allogeneic bone marrow transplantation or prior solid organ transplantation 5. History of idiopathic pulmonary fibrosis (including pneumonitis), drug-induced pneumonitis, idiopathic pneumonitis, organizing pneumonia (i.e., bronchiolitis obliterans, cryptogenic organizing pneumonia), or evidence of active pneumonitis on screening chest computed tomography (CT) scan. Note: History of radiation pneumonitis within the radiation field (fibrosis) is permitted. 6. Positive test for human immunodeficiency virus (HIV) 7. Active hepatitis B (defined as having a positive hepatitis B surface antigen \[HBsAg\] test prior to randomization) or hepatitis C Note: Patients with past hepatitis B virus (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. Patients positive for hepatitis C virus (HCV) antibody are eligible only if polymerase chain reaction testing is negative for HCV ribonucleic acid (RNA). 8. Active tuberculosis 9. Known dihydropyrimidine dehydrogenase (DPD) deficiency. Patients with a reduced DPD activity (CPIC activity score of 1.0-1.5) might participate in the study and receive a reduced dosage of 5-FU after discussion with the lead investigator and sponsor 10. Uncontrolled tumor-related pain; Patients requiring pain medication must be on a stable regimen at study entry 11. Administration of a live, attenuated vaccine within four weeks prior to start of enrollment, or anticipation that such a live attenuated vaccine will be required during the study or within 5 months after the last dose of atezolizumab 12. Prior treatment with CD137 agonists or immune checkpoint blockade therapies, including anti-CTLA4, anti-PD-1, or anti-PD-L1 therapeutic antibodies 13. Treatment with systemic immunostimulatory agents (including but not limited to interferons or interleukin-2) within four weeks or five half-lives of the drug, whichever is longer, prior to study enrollment 14. Treatment with systemic corticosteroids or other systemic immunosuppressive medications within 2 weeks prior to study enrollment. The use of inhaled corticosteroids and mineralocorticoids (e.g., fludrocortisone) is allowed. 15. Significant cardiovascular disease, such as cardiac disease (New York Heart Association Class II or greater), myocardial infarction or cerebrovascular accident within 3 months prior to initiation of study treatment, unstable arrhythmias, or unstable angina. 16. Clinically significant valvular defect 17. History of other malignancy within 3 years prior to screening, with the exception of malignancies with a negligible risk of metastasis or death (e.g. 5-year OS rate \>90%), such as adequately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, localized prostate cancer, ductal carcinoma in situ or Stage I uterine cancer 18. Known central nervous system metastases 19. Peripheral polyneuropathy ≥ NCI CTCAE grade 2 20. Serum albumin \< 2.5 g/dL. 21. Uncontrolled or symptomatic hypercalcemia (ionized calcium \> 1.5 mmol/L, calcium \> 12 mg/dL or corrected serum calcium \> ULN) 22. Serious infection requiring oral or IV antibiotics within 14 days prior to study enrollment 23. Chronic inflammatory bowel disease 24. Clinically significant active gastrointestinal bleeding 25. Major surgical procedure other than for diagnosis within 4 weeks prior to initiation of study treatment 26. Evidence of any other disease, neurologic or metabolic dysfunction, physical examination finding or laboratory finding giving reasonable suspicion of a disease or condition that contraindicates the use of any of the study medications, puts the patient at higher risk for treatment-related complications or may affect the interpretation of study results 27. Participation in another interventional clinical study ≤ 30 days prior to study enrollment or planned participation in such a study at the same time as this study 28. Receipt of an investigational drug within 28 days prior to initiation of study drug 29. Pregnancy or breast feeding or planning to become pregnant within 5 months after the end of treatment. Women of childbearing potential must have a negative serum pregnancy test result within 7 days prior to enrollment.