Gemcitabine, Cisplatin, Nab-paclitaxel (GAP) and Cemiplimab for Locally Advanced Biliary Tract Cancer (BTC)

Inclusion Criteria: * Histologically confirmed diagnosis of biliary tract adenocarcinoma (intra- or extra-hepatic, and gallbladder) * Locally advanced, unresectable BTC without evidence of distant metastatic disease. Patients with surgically unresectable BTC on diagnostic abdominal CT scan or MRI are eligible to participate in the study. Unresectable, locally advanced, but non-metastatic BTC must be confirmed with the designated site radiologist and surgeon at the treating institution (Appendix 2) and must meet at least one of the following criteria: * Tumor involvement of both hepatic lobes and/or vessels * Vascular invasion of the portal vein or main hepatic artery * For perihilar tumors: bilateral hepatic duct involvement up to secondary radicles * Atrophy of one liver lobe with invasion of contralateral vessel and/or bile duct * Extrahepatic organ tumor invasion, except contiguous involvement of the diaphragm * Inadequate estimated liver remnant after surgery If resectability cannot be determined based on CT or MRI, an FDG-PET may be performed, and if it cannot be determined based on imaging alone, surgical exploration is permitted. * Age ≥ 18 years. Because no dosing or adverse event data are currently available on the use of cemiplimab added to gemcitabine, cisplatin, and nab-paclitaxel in participants \<18 years of age, children are excluded from this study. * Treatment naïve; no prior systemic therapy * Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1 * Life expectancy of greater than 3 months. * Have measurable disease, defined as at least one lesion that can be accurately measured in at least one dimension (longest diameter to be recorded for non-nodal lesions and short axis for nodal lesions) as ≥20 mm with conventional techniques or as ≥10 mm with spiral CT scan, MRI, or calipers by clinical exam. See Section 13.3 for more information regarding evaluation of measurable disease. * Adequate hematological and organ function (test results from within 14 days prior to initiation of study treatment): * Absolute Neutrophil Count (ANC) ≥ 1.5 × 10\^9/L without granulocyte colony-stimulating factor support * White blood cell (WBC) count ≥ 2.5 x 10\^9/L (2500/uL) * Lymphocyte count ≥ 0.5 x 10\^9/L (500/uL) * Platelet count ≥ 100 x 10\^9/L (100,000/uL) without transfusion * Hgb ≥ 9.0 g/dL * AST(SGOT)/ALT(SGPT) ≤ 2.5 × institutional upper limit of normal (ULN) * Total bilirubin ≤ 1.5 × ULN, unless in patients with known Gilbert disease (≤ 3 × ULN), or unless elevated secondary to biliary obstruction due to malignancy amenable to decompression prior to administration of investigational therapy * Creatinine within ULN or calculated creatinine clearance (CrCl) ≥ 60 mL/min using the Cockcroft-Gault formula * International Normalized Ratio (INR) and Activated Partial Thromboplastin Time (aPTT) ≤ 1.5 × ULN, except for those on stable anticoagulation for at least two weeks * Liver function Child-Pugh class A or B7, if there is evidence of cirrhosis * Criteria for known hepatitis B and C positive subjects: * Participants who are HBsAg positive are eligible if they have received HBV antiviral therapy for at least 4 weeks and have undetectable HBV viral load prior to enrollment. Participants should remain on antiviral therapy throughout the study and follow local guidelines for HBV anti-viral therapy after the completion of the study. * Participants with history of HCV infection are eligible if HCV viral load is undetectable at screening. Participants must have completed curative antiviral therapy at least 4 weeks prior to enrollment. * Subjects who are HIV positive are eligible if their viral load is undetectable at screening and their CD4+ T cell count is ≥ 200 cells/mm3. Caution should be exercised for subjects on antiretrovirals that may be inhibitors or inducers of CYP2C8 or CYP3A4. * Negative pregnancy test: Women of child-bearing potential must have a negative serum pregnancy test at screening and must agree to use an effective form of contraception from the time of the negative pregnancy test until a minimum of 3 months after the last dose of study drug. Effective forms of contraception include abstinence, hormonal contraceptive (injectable or implantable) in conjunction with a barrier method. Women of non-child-bearing potential must have been postmenopausal for ≥ 1 year or surgically sterile. * Birth control agreement: The effects of cemiplimab on the developing human fetus are unknown. For this reason and because immunotherapy agents and cytotoxic chemotherapy used in this trial are known to be teratogenic, women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry and for the duration of study participation. Should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately. Men treated or enrolled on this protocol must also agree to use adequate contraception prior to the study, for the duration of study participation, and 4 months after completion of treatment. * The participant must understand and be willing to sign a written informed consent document which includes agreement that the participant (or insurance) will be charged gemcitabine, cisplatin, nab-paclitaxel. * The participants must be able to comply with the study protocol, according to the investigator's judgment. Exclusion Criteria: * Histologies other than adenocarcinoma such as mixed hepatocellular carcinoma/cholangiocarcinoma, adenosquamous carcinoma or mixed adenocarcinoma/neuroendocrine carcinoma; ampullary carcinomas are also excluded. * Has initially resectable disease or distant metastasis, including distant lymph nodes. Resectable BTC include the following: absence of retropancreatic and paraceliac nodal metastases or distant liver metastases, absence of in