This study is a randomized, Double-Blind, multi-center Phase III clinical study, aimed to evaluate the efficacy and safety of SHR1701 combined with chemotherapy in the treatment of Previously Untreated, Advanced or Metastatic Gastric or Gastroesophageal Junction Cancer. For Part 1 study，the tolerability of SHR-1701 will be evaluated and determine the recommended dose for Part 2.For Part 2 study, all enrolled patients will be randomized to 2 groups and continuously treated until the end criteria of treatment was met.

Inclusion Criteria:

1. Pathologically confirmed diagnosis of locally advanced unresectable or metastatic gastric or gastroesophageal junction (GEJ)adenocarcinoma.
2. HER2 overexpression or amplification negative.
3. Female or male, 18 years of age or above.
4. Eastern Cooperative Oncology Group (ECOG) performance status of 0 - 1.
5. Patients who are willing and able to provide the signed informed consent form, willing and able to comply with all the scheduled visits, study treatment, laboratory tests, and other study procedures.

Exclusion Criteria:

1. Squamous cell carcinoma, undifferentiated carcinoma, or other histological types of gastric cancer.
2. Presence of inadequately treated CNS metastases, or uncontrolled or symptomatic active CNS metastases ，leptomeningeal disease, and/or rapid progression.
3. Presence of uncontrolled pleural effusion or ascites despite puncture drainage within 14 days prior to randomization.
4. More than 20% weight loss within 2 months prior to randomization.
5. Diagnosed with other malignant tumors within 5 years prior to enrollment.
6. Presence of any active, known or suspected autoimmune disease.
7. Prior treatment with TGF-β inhibitor, anti-PD-1/PD-L1 antibodies, anti-PD-L2 antibodies, anti-CD137 antibodies, anti-CTLA-4 antibodies, or other drugs/antibodies.
8. Severe, unhealed, or dehisced wounds and active ulcers or untreated fractures.