Pembrolizumab (MK-3475) Plus Gemcitabine/Cisplatin Versus Placebo Plus Gemcitabine/Cisplatin for First-Line Advanced and/or Unresectable Biliary Tract Carcinoma (BTC) (MK-3475-966/KEYNOTE-966)

Inclusion Criteria * Has histologically confirmed diagnosis of advanced (metastatic) and/or unresectable (locally advanced) biliary tract cancer (intra-or extrahepatic cholangiocarcinoma or gallbladder cancer) * Has measurable disease based on Response Evaluation Criteria in Solid Tumors (RECIST 1.1), as determined by the site investigator * Participants with a history of hepatitis B or hepatitis C can be enrolled if they meet study criteria * Is able to provide archival tumor tissue sample or newly obtained core or excisional biopsy of a tumor lesion * Has a life expectancy of greater than 3 months * Has adequate organ function Exclusion Criteria * Has had previous systemic therapy for advanced (metastatic) or unresectable (locally advanced) biliary tract cancer (intra-or extra hepatic cholangiocarcinoma or gallbladder cancer) * Has ampullary cancer * Has small cell cancer, neuroendocrine tumors, lymphoma, sarcoma, mixed tumor histology and/or mucinous cystic neoplasms * Has received prior therapy with an anti-programmed cell death 1 (anti-PD-1), anti- programmed cell death ligand 1 or 2 (anti-PD-L1, anti-PD-L2) agent or with an agent directed to another stimulatory or coinhibitory T-cell receptor (e.g., cytotoxic T-lymphocyte-associated protein 4 \[CTLA-4\], OX-40, CD137) * Has a known history of, or any evidence of, central nervous system (CNS) metastases and/or carcinomatous meningitis, as assessed by local site investigator * Has had an allogenic tissue/solid organ tr…