An Open-Label, Multi-Drug, Multi-Centre, Phase II Study to Evaluate the Efficacy, Safety, Tolerability, Pharmacokinetics, and Immunogenicity of Novel Combinations in Participants With Locally Advanced Unresectable or Metastatic Gastric or Gastroesophageal Junction Adenocarcinoma

Inclusion Criteria: * 18 years or older at the time of signing the ICF. * Body weight \> 35 kg. * Previously untreated for unresectable or metastatic gastric or gastroesophageal junction adenocarcinoma. * Has measurable target disease assessed by the Investigator based on RECIST 1.1. * ECOG PS zero or one. * Life expectancy of at least 12 weeks. * Adequate organ and bone marrow function. * Has central lab confirmed Claudin18.2 status at screening from archival tumour collected within past 24 months or from a fresh biopsy when Substudy 3, Substudy 4 i s open for recruitment. Exclusion Criteria: * Participants with HER2-positive (3+ by IHC, or 2+ by IHC and positive by in situhybridisation) or indeterminate gastric or GEJ carcinoma. * Untreated or progressive CNS metastatic disease, any leptomeningeal disease, or cord compression. * Participants with ascites which cannot be controlled with appropriate interventions. * Active infectious diseases, including tuberculosis, HIV infection, or hepatitis B/C. * Uncontrolled intercurrent illness. * Active or prior documented autoimmune or inflammatory disorders requiring systemic treatment with steroids or other immunosuppressive treatment. * History of another primary malignancy. * Previous treatment with an immune-oncology agent. * Previous treatment with any modalities of Claudin18.2 target therapy or MMAE exposure (when Substudy 3, Substudy 4 is open for recruitment).