Study of Denikitug (GS-1811) Given Alone or With Nivolumab or Chemotherapy in Adults With Metastatic Gastric, Gastroesophageal Junction (GEJ), and Esophageal Adenocarcinomas

Key Inclusion Criteria: * Histologically or cytologically confirmed diagnosis of locally advanced, unresectable, or metastatic gastric, gastroesophageal junction (GEJ), or esophageal adenocarcinoma (EAC). * Human epidermal growth factor receptor 2 (HER2)-negative status, as determined by local assessment using a validated immunohistochemistry assay, in situ hybridization or other amplification testing. * Has had disease progression during or after first line of systemic therapy for advanced or metastatic gastric, GEJ, or EACs, which must have included at least one of the following: 1. Platinum- and fluoropyrimidine-based chemotherapy. 2. Therapy with an anti-programmed cell death protein 1 (PD1) or anti-programmed cell death ligand 1 (anti-PD-L1) monoclonal antibody (patients with PD-L1-positive tumors must have received prior PD-1/PD-L1-based therapy). 3. Zolbetuximab or other Claudin-18 (CLDN18).2-targeted therapy, if indicated based on biomarker status. * Eastern Cooperative Oncology Group (ECOG) performance status score of 0-1. * Have adequate organ function. * Male individuals and female individuals of childbearing potential who engage in heterosexual intercourse must agree to use methods of contraception. Key Exclusion Criteria: * Active or history of autoimmune disease requiring systemic treatment within 2 years, inflammatory bowel disease (IBD) (Crohn's/ulcerative colitis), celiac disease, or noninfectious enteritis/colitis. (Physiologic hormone replacement not considered systemic treatment). * History or current noninfectious pneumonitis/interstitial lung disease, including radiation-induced pneumonitis requiring steroids or active/recurrent pneumonitis of any etiology. * Documented microsatellite instability-high (MSI-H) or deficient mismatch repair (dMMR) disease by local polymerase chain reaction (PCR) (microsatellite status) and/or informed consent form (ICH) (mismatch repair (MMR)) assay * (For Part 2 only) Has known history of peripheral neuropathy ≥ Grade 2 (per National Cancer Institute(NCI)-Common Tenninology Criteria for Adverse Events (CTCAE) Version 5.0). * (For Part 2 only) Known coagulopathy that increases the risk of bleeding, bleeding diatheses. Any other Grade 3 or higher hemorrhage/bleeding event within 28 days prior to enrollment. Prior/Concurrent Therapy or Clinical Study Experience * Prior treatment with DEN or other C-C chemokine receptor 8 (CCR8)-targeted agents. * Prior Lonsurf (trifluridine-tipiracil) or paclitaxel (PAC)-based regimens in the first-line setting for advanced/metastatic gastroesophageal adenocarcinoma. * Any systemic therapy (including investigational) targeting vascular endothelial growth factor (VEGF) or VEGF receptor (VEGFR) signaling pathways. * Anticancer biologic within 4 weeks, orchemotherapy, targeted small molecule, or radiation therapy within 2 weeks prior to enrollment with unresolved adverse events (AE)s (Grade \>2). (Observational study participants are eligible). * Prior allogenic tissue/solid organ or stem cell transplantation. (Exception: corneal transplant not requiring systemic immunosuppression is allowed). Note: Other protocol defined Inclusion/Exclusion criteria may apply.