A Clinical Study To Evaluate Camrelizumab (SHR-1210) Plus Capecitabine and Oxaliplatin Followed by Sequential Treatment With Camrelizumab Plus Apatinib Mesylate in Advanced or Metastatic Gastric Cancer (GC) or Gastroesophageal Junction Cancer (GEJ) Without Prior Systemic Therapy

Inclusion Criteria:

* Has histologically- or cytologically-confirmed diagnosis of locally advanced unresectable or mestastatic adenocarcinoma of stomach or the esophagogastric junction (GEJ)
* Age ≥ 18 years old, male or female
* NO previous therapy for advanced/metastatic disease of GC/GEJ (including HER-2 inhibitor). Subjects with previous adjuvant/neo-adjuvant therapy completed more than 6 months can be enrolled.
* Has measurable disease per RECIST 1.1
* Eastern Cooperative Group (ECOG) performance status of 0 to 1
* Has adequate organ function
* Females of childbearing potential (FOCBP), who are not surgically sterile or postmenopausal, must conduct pregnancy test (serum or urine) within 7 days before enrollment, and must not be pregnant or breast-feeding women. If the result is negative, she must agree to use adequate contraception during the experiment and 3 months after the last administration of the test drugs. And non-sterilized males who are sexually active must agree to use adequate contraception during the experiment and 3 months after the last administration of the test drugs.

Exclusion Criteria:

* Has known HER2-positive status
* Has known active central nervous system metastatases
* Has received a live vaccine within 4 weeks prior to the first dose of study treatment
* With any acitve autoimmune disease or history of autoimmune disease, including but not limited to the following: hepatititis, pneumonitis, uveitis, colitis (inflammatory bowel disease), hypophysitis, vasculitis, nephritis, hyperthyroidism, and hypothyroidism, except for subjects with vitiligo or resolved childhood asthma/atopy. Asthma that requires intermittent use of bronchodilators or other medical intervention should also be excluded.
* Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-CTLA-4 antibody, or a VEGFR inhibitor.
* Clinically significant cardiovascular and cerebrovascular diseases, including but not limited to severe acute myocardial infarction within 6 months before enrollment, unstable or severe angina, Congestive heart failure (New York heart association (NYHA) class \> 2), orventricular arrhythmia which need medical intervention.
* Hypertension and unable to be controlled within normal level following treatment of anti-hypertension agents: systolic blood pressure \> 140 mmHg, diastolic blood pressure \> 90 mmHg.