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).