This prospective observational study aims to evaluate the efficacy and safety of immune checkpoint inhibitors as first-line and neoadjuvant therapy for advanced gastric cancer, while also investigating relevant biomarkers to better understand their role in immunotherapy outcomes

Inclusion Criteria:

1. Age 18 years old or above
2. Patients with advanced gastric cancer or locally advanced gastric cancer
3. Have not received any previous anti-tumor therapy
4. Patients expected to receive immunotherapy for first-line or neoadjuvant therapy
5. Eastern Cooperative Oncology Group (ECOG) performance status of 0-1
6. Adequate organ function

Exclusion Criteria:

1. Patients with contraindications to immunotherapy
2. Have received anti-tumor treatments such as immunotherapy and chemotherapy
3. Have a history of active immune deficiency or autoimmune diseases, including HIV positive test, or have other acquired or congenital immune deficiency diseases, or have a history of organ transplantation or autoimmune diseases
4. Severe chronic or active infection requires systemic antibacterial, antifungal, or antiviral treatment, including tuberculosis infection. Have a history of active tuberculosis infection ≥ 1 year before recruitment should also be excluded, unless proved has been completed appropriate treatment
5. History of allogeneic stem cell transplantation or organ transplantation

Patients with pathologically defined, locally inoperable or advanced gastric cancer who have not received any previous anti-tumor therapy and are expected to receive immunotherapy.