A Study to Evaluate the Safety and Tolerability of AB680 in Participants With Gastrointestinal Malignancies

Key Inclusion Criteria: * Histologically or cytologically confirmed metastatic pancreatic adenocarcinoma * Naïve to any prior treatment, including chemotherapy, biological therapy, or targeted therapy for metastatic disease * Prior adjuvant therapy (including chemotherapy and/or radiotherapy) for pancreatic adenocarcinoma is permitted if neoadjuvant or adjuvant therapy was completed at least 6 months prior to study enrollment. Prior adjuvant therapy may include nab- paclitaxel or gemcitabine * Participants initially diagnosed with locally advanced pancreatic cancer who have undergone chemotherapy then resection and had no evidence of disease are eligible if relapse of metastatic disease has occurred and if the last dose of chemotherapy was received more than 6 months before study entry * Must have at least 1 measurable lesion per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1. The measurable lesion must be outside of a radiation field if the participant received prior radiation * Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1 * Confirmation that an archival tissue sample is available; if not, a new biopsy of a tumor must be obtained * Prior radiation therapy for metastatic disease must have been completed * Immunosuppressive doses of systemic medications, such as corticosteroids or absorbed topical corticosteroids (doses \> 10 mg/day prednisone or equivalent) must be discontinued at least 2 weeks (14 days) before investigational product administration. Physiologic doses of corticosteroids (≤ 10 mg/day of prednisone or its equivalent) or short pulses of corticosteroids (≤ 3 days) may be permitted * Prior surgery that required general anesthesia or other major surgery as defined by the Investigator must be completed at least 4 weeks before investigational product administration * Negative tests for hepatitis B surface antigen, hepatitis C virus antibody (or hepatitis C qualitative ribonucleic acid \[RNA; qualitative\]), and human immunodeficiency virus (HIV)-1 and HIV-2 antibody at screening * Adequate organ and marrow function Key Exclusion Criteria: * Significant cardiovascular disease (NYHA Class III-IV), myocardial infarction or cerebrovascular accident within 12 months of the first dose of investigational agent or history of arterial thromboembolic event, uncontrolled hypertension, unstable arrhythmia, or unstable angina within 3 months or venous thromboses within 1 month of the first dose of investigational agent. * Any active autoimmune disease or a documented history of autoimmune disease or history of a syndrome that required systemic steroids or immunosuppressive medications, except for vitiligo or resolved childhood asthma/atopy. Participants with asthma who require intermittent use of bronchodilators (such as albuterol) will not be excluded from this study * History of a syndrome that required systemic steroids or immunosuppressive medications, except for vitiligo or resolved childhood asthma/atopy. Participants with asthma who require intermittent use of bronchodilators (such as albuterol) will not be excluded from this study * Prior malignancy active within the previous year except for locally curable cancers that have been apparently cured, such as basal or squamous cell skin cancer, superficial bladder cancer, or carcinoma in situ of the cervix, breast, or prostate cancer * Has not recovered (ie, ≤ Grade 1 or baseline) from a non-hematologic AEs due to a previously administered agent, except ≤ Grade 2 alopecia or ≤ Grade 2 neuropathy. Note: Other protocol defined Inclusion/Exclusion criteria may apply.