A Phase Ib/II Study of ATG-022 Plus Pembrolizumab With/Without Chemotherapy in Participants With Claudin (CLDN) 18.2-positive, HER2-negative, Unresectable or Metastatic Gastric or Gastroesophageal Junction AdenocarcinomaUnresectable or Metastatic Gastric or Gastroesophageal Junction Adenocarcinoma

Inclusion Criteria: 1. Provision of signed and dated, written informed consent prior to any study-specific procedures, sampling, and analyses. 2. Aged ≥18 years as of the date of consent. 3. Histological or cytological confirmation of gastric cancer or gastroesophageal junction adenocarcinoma with CLDN 18.2 positive, HER2-negative and PD-L1 positive expression. 4. Archival tumor tissue sample within 36 months prior to participating in the study or newly obtained biopsy of a tumor lesion not previously irradiated should be provided for testing of CLDN 18.2 and PD-L1 expression for determining the criteria. 5. At least 1 measurable lesion per Response Evaluation Criteria In Solid Tumors (RECIST) v1.1 by investigators. 6. Estimated life expectancy of a minimum of 12 weeks. Exclusion Criteria: 1. Known active central nervous system metastases and/or carcinomatous meningitis. Participants with previously treated brain metastases may participate provided they are radiologically stable (i.e., without evidence of progression) for at least 4 weeks as confirmed by repeat imaging performed during the study screening, are clinically stable and have not required steroid treatment for at least 14 days before the first dose of study intervention. 2. Prior exposure to CLDN 18.2 ADC, CLDN 18.2 chimeric antigen receptor T-cell immunotherapy or agents containing MMAE. 3. Prior systemic anticancer therapy including investigational agents within 4 weeks before the first dose of study treatment or within a period during which the investigational product or systemic anticancer treatment has not been cleared from the body (e.g., a period of 5 'halflives'), whichever is the most appropriate as judged by the investigator. 4. Received any prior immunotherapy and was discontinued from that treatment due to a Grade 3 or higher irAE (except endocrine disorders that can be treated with replacement therapy) or was discontinued from that treatment due to Grade 2 myocarditis or recurrent Grade 2 pneumonitis. 5. Prior radiotherapy within 4 weeks of start of study intervention, or has radiation-related toxicities, requiring corticosteroids. Two weeks or fewer of palliative radiotherapy for non- central nervous system disease is permitted. The last radiotherapy treatment must have been performed at least 7 days before the first dose of study intervention 6. Prior a live or live-attenuated vaccine within 30 days before the first dose of study intervention. Administration of killed vaccines is allowed. 7. Prior major surgery (excluding placement of vascular access) within 28 days of the first dose of study treatment or minor surgical procedures≤7 days. No waiting is required following implantable port and catheter placement.