This is a non-randomized, multicenter, open-label, single-arm Phase II study in patients with inoperable histologically proven inoperable locally advanced or metastatic gastric cancer. Eligible patients must be therapy naïve and have received no previous chemotherapy or immune therapy for their inoperable gastric cancer. Neo/Adjuvant Chemotherapy or adjuvant Chemo/Radiotherapy are allowed. A total of 40 evaluable patients will be recruited and evaluated for efficacy and safety of a sequential chemoimmunotherapy combination regime.