A Dose Optimization/Expansion Study of SAR445877 in Adult Chinese Participants With Advanced Gastric or Gastroesophageal Junction Cancer

Inclusion Criteria: Age \- Participant must be at least 18 years of age inclusive (or country's legal age of majority if \>18 years), at the time of signing the informed consent. Cancer diagnosis: * Histologically or cytologically confirmed diagnosis of advanced unresectable or metastatic GC or Siewert Type 2 \& 3 GEJ. * Participants with unknown HER2/neu status must have their HER2/neu status determined locally. Participants with HER2/neu negative are eligible. Participants with HER2/neu positive tumors must have documentation of disease progression on treatment containing an approved HER2 targeted therapy to be eligible. Prior anticancer therapy: \- Participants should have failed or relapsed after at least 1 prior line of treatment which may or may not include an anti-PD1/PD-L1-based treatment or anti-Claudin 18.2 based treatment depending on local standard of care. Measurable Disease: \- At least 1 measurable lesion per RECIST 1.1 criteria. Exclusion Criteria: Medical conditions * Eastern Cooperative Oncology Group(ECOG)performance status of ≥2. * Predicted life expectancy ≤3 months. * Diagnosed of any other malignancies, either progressing or requiring active treatments, within 2 years prior to enrollment. * Active brain metastases or leptomeningeal metastases. * Known microsatellite instability-high (MSI-H) or mismatch repair-deficient (dMMR) tumor. * History of treatment-related immune-mediated (or immune-related) AEs from immune- modulatory agents (including but not limited to anti-PD1/PD-L1 agents and anti cytotoxic T lymphocyte associated protein 4 monoclonal antibodies) that caused permanent discontinuation of the agent, or that were Grade 4 in severity, or have not resolved to Grade ≤1. * Has any condition requiring ongoing/continuous corticosteroid therapy (\>10 mg prednisone/day or an anti-inflammatory equivalent) within 1 week prior to the first dose of the study medicine. * Any clinically significant cardiac (including valvular) or vascular (thromboembolic disorders) disease, within 6 months prior to the first IMP administration. * Ongoing or recent (within 2 years) evidence of significant autoimmune disease that required treatment with systemic immunosuppressive treatments, which may suggest risk for immune-related AEs (irAEs). * Has a known history or any evidence of interstitial lung disease or active, non-infectious pneumonitis within 3 years prior to the first dose of the study drug. * Organ transplant requiring immunosuppressive treatment. * Uncontrolled or active infection with human immunodeficiency virus (HIV ), hepatitis B, or hepatitis C infection, or has a diagnosis of immunodeficiency. Note: Other Inclusion/Exclusion criteria may apply. The above information is not intended to contain all considerations relevant to a participant's potential participation in a clinical trial.