This is a phase 1b study designed to evaluate the safety, tolerability and efficacy of IBI363 in combination with oxaliplatin and capecitabine (XELOX) in first-line treatment of unresectable advanced or metastatic gastric and gastroesophageal junction adenocarcinoma.

Inclusion Criteria:

1. Male or female subjects, ≥ 18 years and ≤75 years.
2. Subjects with unresectable advanced or metastatic gastric and gastroesophageal junction adenocarcinoma without prior systematic treatment.
3. Subjects with at least one measurable lesion according to RECIST v1.1.
4. Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 or 1.
5. Expected survival time ≥ 3 months.

Exclusion Criteria:

1. Women who are pregnant or lactating, or intending to become pregnant before, during, or within 6 months after the last dose of study drug.
2. Active uncontrolled bleeding or a known bleeding diathesis.
3. Subjects with history of or known active seizure disorder, brain metastases, spinal cord compression, or carcinomatous meningitis, or new evidence of brain or leptomeningeal disease.