This study is conducted in patients with advanced metastatic gastric cancer including gastroesophageal junction cancer(patients with HER2 negative and PD-L1 CPS≥5).Patiens will recevie Serplulimab plus chemotherapy ((oxaliplatin+capecitabine) as first-line treatment. After PD，patients will randomly 2：1 assigned to treatment：one group will receive Serpluimab with Paclitaxel,Apatinib；another group will receive Paclitaxel with or without Ramucirumab.All of eligible patients will receive study drug treatment until loss of clinical benefit, unacceptable toxicity, death, withdrawal of informed consent

stage one ：Serplulimab 3mg/kg,d1,q2w or 4.5mg/kg,d1,q3w+oxaliplatin130mg/m2 iv.gtt d1+capecitabine 1000mg/m2 p.o.b.i.d d1~d14,q3w

stage two:Serplulimab 3mg/kg,d1,q2w or 4.5mg/kg,d1,q3w+Apatinib 250mg/qd+Paclitaxel 135~175mg/m2 iv.gtt,d1,q3w

Paclitaxel 135~175mg/m2 iv.gtt,d1,q3w±Ramucirumab 8mg/kg,d1,q2w

Inclusion Criteria:

1. Volunteer to participate in this clinical study; completely understand and know this study as well as sign the informed consent form (ICF); be willing to follow and be able to complete all study procedures;
2. Age ≥ 18 years and ≤ 75 years when ICF is signed;
3. Unresectable locally advanced, or metastatic gastric cancer including gastroesophageal junction cancer, and histopathologically confirmed diagnosis of adenocarcinoma;
4. never received systemic anti-tumor drug therapy before；
5. HER2 negative and PD-L1 CPS≥5;
6. Measurable lesion according to RECIST v1.1 by IRRC;
7. ECOG score 0-1;

Exclusion Criteria:

1. Has other active malignancies within 5 years before the first administration of the study drug;
2. Plan to or have previously received organ or bone marrow transplantation;
3. Uncontrollable pleural effusion, pericardial effusion or ascites requiring repeated drainage;
4. Have received any research drugs within 14 days before the first use of the study drugs.