Study to Evaluate the Efficacy and Safety of Andecaliximab Combined With Nivolumab Versus Nivolumab Alone in Adults With Unresectable or Recurrent Gastric or Gastroesophageal Junction Adenocarcinoma

Key Inclusion Criteria: * Histologically confirmed inoperable locally advanced or metastatic adenocarcinoma of the stomach or GEJ which have progressed on at least 1 prior systemic therapy or line of treatment for unresectable/metastatic disease * Eastern Cooperative Oncology Group (ECOG) performance score of ≤ 1 * Measurable disease according to Response Criteria in Solid Tumors (RECIST) v1.1 * Tumor sites that can be accessed for repeat biopsies * Archival tumor tissue, preferably obtained from the most recent available biopsy; there must be adequate tissue for a Cochran-Mantel Haenszel (CMH) test stratified by programmed death ligand 1 (PD-L1) stratification test, as assessed by central pathologist * Individuals not receiving anticoagulant medication must have an international normalized ratio (INR) ≤ 1.5 and activated partial thromboplastin (aPTT) ≤ 1.5 x upper limit of normal (ULN) * Required baseline laboratory data as outlined in protocol Key Exclusion Criteria: * Individuals who have received only neoadjuvant or adjuvant therapy for gastric adenocarcinoma * Radiotherapy within 28 days of randomization * Uncontrolled intercurrent illness as outlined in protocol * History of a concurrent or second malignancy except for those outlined in protocol * Major surgery, within 28 days of first dose of study drug * Known positive status for human immunodeficiency virus (HIV) * Known acute or chronic-active infection with hepatitis B virus (HBV) or hepatitis C virus (HCV) * Chronic daily treatment with oral corticosteroids (dose of \> 10 mg/day prednisone equivalent) or other immunosuppressive medications within 14 days of randomization * Known or suspected central nervous system metastases * Documented myocardial infarction or unstable/uncontrolled cardiac disease within 6 months of randomization * Serious systemic fungal, bacterial, viral, or other infection that is not controlled or requires intravenous antibiotics * Current or history of pneumonitis or interstitial lung disease * Active known or suspected autoimmune disease with exceptions noted in protocol. * History of bone marrow, stem cell, or allogenic organ transplantation NOTE: Other protocol defined Inclusion/ Exclusion criteria may apply.