This study is conducted in patients with advanced metastatic gastric cancer including gastroesophageal junction cancer. This study includes two arms: A and B. Arm A (patients with HER2 negative and PD-L1 CPS≥5 ) will receive HLX07 combination therapy with HLX10 and chemotherapy (oxaliplatin+capecitabine) as first-line treatment. Arm B will receive HLX07 monotherapy as third-line or above treatment. All of eligible patients will receive study drug treatment until loss of clinical benefit, unacceptable toxicity, death, withdrawal of informed consent (whichever occurs first, HLX10 treatment up to 2 years).

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. Arm A: never received systemic anti-tumor drug therapy before, with HER2 negative and PD-L1 CPS≥5; Arm B: failed to prior systemic anti-tumor therapy (at least 2 lines);
5. Measurable lesion according to RECIST v1.1 by IRRC;
6. ECOG score 0-1;
7. Expected survival 12 weeks.

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. Arm A: previously received antibody drugs against immune checkpoints (such as PD-1, PD-L1, CTLA4, etc.) and / or antibody drugs against EGFR; Arm B: previously received antibody drug treatment against EGFR;
5. Have received any research drugs within 14 days before the first use of the study drugs.