Novel Combinations in Participants With Locally Advanced Unresectable or Metastatic Gastric or Gastroesophageal Junction Adenocarcinoma

Inclusion Criteria:

1. 18 years or older at the time of signing the ICF.
2. Body weight > 35 kg.
3. Previously untreated for unresectable or metastatic gastric or gastroesophageal junction adenocarcinoma.
4. Has measurable target disease assessed by the Investigator based on RECIST 1.1.
5. ECOG PS zero or one.
6. Life expectancy of at least 12 weeks.
7. Adequate organ and bone marrow function.
8. 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 or Substudy 6 is open for recruitment.

Exclusion Criteria:

1. Participants with HER2-positive (3+ by IHC, or 2+ by IHC and positive by in situhybridisation) or indeterminate gastric or GEJ carcinoma.
2. Untreated or progressive CNS metastatic disease, any leptomeningeal disease, or cord compression.
3. Participants with ascites which cannot be controlled with appropriate interventions.
4. Active infectious diseases, including tuberculosis, HIV infection, or hepatitis A/B/C.
5. Uncontrolled intercurrent illness.
6. Active or prior documented autoimmune or inflammatory disorders requiring systemic treatment with steroids or other immunosuppressive treatment.
7. History of another primary malignancy.
8. Previous treatment with an immune-oncology agent.
9. Previous treatment with any modalities of Claudin18.2 target therapy or MMAE exposure (when Substudy 3, Substudy 4, or Substudy 6 is open for recruitment).