Perioperative Gemcitabine, Cisplatin, and Pembrolizumab in Potentially Resectable Biliary Tract Cancers

Inclusion Criteria:

* Must have a newly diagnosed, biopsy-proven biliary tract cancer (BTC) including gallbladder, intrahepatic, extrahepatic, and hilar cholangiocarcinoma.
* Resectable BTC (biliary tract cancer)
* Measurable disease per RECIST 1.1 as determined by the investigator.
* Age ≥18 years.
* ECOG (Eastern Cooperative Oncology Group) performance status ≤1 or Karnofsky ≥80
* Patients must have adequate organ and marrow function defined by study-specified laboratory tests.
* Patients must have adequate liver function defined by study-specified laboratory tests.
* Patients with chronic or acute HBV or HCV infection must have disease controlled prior to enrollment.
* Women of childbearing potential (WOCBP) must have a negative urine or serum pregnancy test.
* For both Women and Men, must use acceptable form of birth control while on study.
* Ability to understand and willingness to sign a written informed consent document.

Exclusion Criteria:

* Receiving, or previously received, any systemic chemotherapy, or investigational agent for BTC.
* Has received prior radiotherapy within 2 weeks of start of study intervention.
* Patients with a history of prior treatment with anti-PD-1 and anti-PD-L1.
* Have been diagnosed with another cancer or myeloproliferative disorder whose natural history or treatment has the potential to interfere with safety or efficacy assessment of this study's investigational drugs.
* Has a known history of Human Immunodeficiency Virus (HIV)/AIDS
* Has active co-infection with HBV and HDV.
* Has a diagnosis of immunodeficiency.
* Has active autoimmune disease that has required systemic treatment in the past 2 years.
* Systemic or topical corticosteroids at immunosuppressive doses.
* Prior allogeneic stem cell transplantation or organ transplantation.
* Prior tissue or organ allograft or allogeneic bone marrow transplantation, including corneal transplants.
* Uncontrolled intercurrent active medical and/or psychiatric illness/social psychosocial problems that that would limit compliance with study requirements.
* Uncontrolled intercurrent illness including, but not limited to, uncontrolled infection, symptomatic congestive heart failure, unstable angina, cardiac arrhythmia, metastatic cancer, or psychiatric illness/social situations that would limit compliance with study requirements.
* Evidence of clinical ascites.
* Has a history of (non-infectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease.
* Previously identified allergy or hypersensitivity to monoclonal antibodies or any component of the study treatment formulations.
* Pregnant or breastfeeding.
* WOCBP and men with female partners (WOCBP) who are not willing to use contraception.
* Subjects unable to undergo venipuncture and/or tolerate venous access.
* Patient is at the time of signing informed consent a regular user (including "recreational use") of any illicit drugs or had a recent history (within the last year) of substance abuse (including alcohol).