Testing Ivonescimab in Combination With Chemotherapy in Advanced or Metastatic Gastric/Gastroesophageal Adenocarcinoma

Inclusion Criteria: 1. Signed a written informed consent form prior to any trial specific procedures. Note: If the patient is physically unable to provide their written consent, a trusted person of their choice, independent of the Investigator or the Sponsor, can confirm the patients consent in writing. 2. Histologically or cytologically proven gastric cancer (GC) or esophagogastric junction cancer adenocarcinoma (EGJC) 3. Metastatic or locally advanced non resectable (stage IV) disease. 4. Presence of at least one measurable lesion as assessed by the investigator according to RECIST v1.1. 5. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. 6. Age ≥18 years. 7. For patients without actionable biomarker (HER2 and Claudin 18.2-negative) except for PD-L1, no prior treatment for advanced disease (cohort 1). For patients with at least one of the following actionable biomarker (PD-L1 CPS≥1, and/or HER2-positive, and/or Claudin 18.2-positive), who had received only one prior line of treatment for advanced disease (cohort 2). 8. Adequate hematological function: absolute neutrophil count (ANC) ≥1.5 × 10⁹/L, platelet count ≥100 × 10⁹/L, and hemoglobin ≥9 g/dL. Note: Blood transfusion or growth factor therapy should not be performed within 7 days prior to the screening hematology analysis. 9. Adequate renal function: estimated creatinine clearance ≥50 mL/min according to the Cockcroft-Gault formula, or estimated glomerular filtration rate (eGFR) value ≥50 mL/min using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation, and urine protein \< 2+ or 24-hour urine protein quantification \< 1.0 g. 10. Adequate liver function: total bilirubin level ≤1.5 × the upper limit of normal (ULN) range (≤3 x ULN for patients with liver metastases or confirmed/suspected Gilbert syndrome), and aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels ≤2.5 × ULN (AST and ALT ≤5 x ULN when documented liver metastasis). 11. Coagulation: prothrombin time (PT) or international normalized ratio (INR) ≤ 1.5 x ULN, and partial prothrombin time (PTT) or activated partial thromboplastin time (aPTT) ≤ 1.5 × ULN (unless abnormalities are unrelated to coagulopathy).This applies only to patients who are not on therapeutic anti-coagulation. Patients receiving therapeutic anti-coagulation should be on a stable dose. 12. Women of childbearing potential (WOCBP) having sex with an unsterilized male partner and unsterilized males having sex with a female partner of childbearing potential, must agree to use an effective method of contraception for the duration of trial participation and as required after completing study treatment (120 days after the last dose of ivonescimab). Men must also agree to not donate sperm and women must agree to not donate oocytes during the specified period. 13. WOCBP must have a negative serum pregnancy test performed within 3 days before inclusion and a negative urine pregnancy test on the day of first dose, prior to treatment administration. 14. Willing and able to comply with the protocol for the duration of the study including scheduled visits, treatment plan, laboratory tests, and other study procedures 15. Affiliation to the Social Security System (or equivalent) Exclusion Criteria: 1. Previous or concurrent cancer that is distinct in primary site or histology from gastroesophageal cancer within 2 years prior to study inclusion, except for curatively treated cervical cancer in situ, non-melanoma skin cancer and superficial bladder tumors \[Ta (non-invasive tumor), Tis (carcinoma in situ) and T1 (lamina propria invasion)\]. 2. Patients with high microsatellite instability (MSI-H) or mismatched repair disease (dMMR) tumor. 3. Enteral intake \< 1500 kcal /d and or a weight loss \> 15% of total body weight within the 6 months 4. Toxicities from previous treatment not resolved to grade ≤ 1 (according to the version 5.0 of the National Cancer Institute - Common terminology criteria for adverse events \[NCI-CTCAE v5.0\]) before treatment start with the exception of alopecia. 5. Major surgical procedures or serious trauma within 4 weeks prior to treatment start, or plans for major surgery within 4 weeks after the first dose (as determined by the investigator). Minor local procedures (excluding central venous catheterization and port implantation) within 3 days prior to treatment start. 6. History of bleeding tendencies or coagulopathy and/or clinically significant bleeding symptoms or risk within 4 weeks prior to inclusion, including but not limited to: 1. Hemoptysis (defined as coughing up ≥ 0.5 teaspoon of fresh blood or small blood clots), Note: transient hemoptysis associated with diagnostic bronchoscopy is allowed. 2. Nasal bleeding /epistaxis (bloody nasal discharge is allowed), 3. Current use of prophylactic or full-dose anticoagulants or anti-platelet agents for therapeutic purposes that is not stable prior to treatment start is not allowed. The use of full-dose anticoagulants is permitted as long as the international normalized ratio (INR) or activated partial thromboplastin time (aPTT) is within therapeutic limits according to the medical standard of the enrolling institution. 7. Poorly controlled hypertension with repeated systolic blood pressure ≥ 150 mmHg or diastolic blood pressure ≥ 100 mmHg after oral antihypertensive therapy. 8. History of major diseases before inclusion, specifically: 1. Unstable angina, myocardial infarction, congestive heart failure (New York Heart Association classification ≥ grade 2) or unstable vascular disease (eg, aortic aneurysm at risk of rupture, Moyamoya disease) that required hospitalization within 12 months prior to inclusion, or other cardiac impairment that may affect the safety evaluation of the study drug (eg, poorly controlled arrhythmias, myocardial ischemia), 2. History of esophageal gastric varices, severe ulcers, wounds that do not heal, abdominal fistula, intra-abdominal a