A Safety and Efficacy Study of ZW25 (Zanidatamab) Plus Combination Chemotherapy in HER2-expressing Gastrointestinal Cancers, Including Gastroesophageal Adenocarcinoma, Biliary Tract Cancer, and Colorectal Cancer

Inclusion: * Disease diagnosis: * Part 1: * GEA: Unresectable, locally advanced, recurrent or metastatic HER2-expressing GEA (IHC 3+ or 2+ with or without gene amplification based upon local assessment or central assessment) * BTC: Unresectable, locally advanced, recurrent or metastatic HER2-expressing BTC (including intrahepatic cholangiocarcinoma \[ICC\], extrahepatic cholangiocarcinoma \[ECC\], or gallbladder cancer \[GBC\]) (IHC 3+ with or without gene amplification; or IHC 0, 1+ or 2+ with gene amplification, based upon central assessment) * CRC: Unresectable, locally advanced, recurrent or metastatic HER2-expressing CRC (IHC 3+ with or without gene amplification; or IHC 0, 1+ or 2+ with gene amplification, based upon central assessment). Patients will be required to be extended RAS (KRAS and NRAS) and BRAF wild-type based upon central assessment. * Part 2: * GEA: Unresectable, locally advanced, recurrent or metastatic HER2-expressing GEA (IHC 3+, or IHC 2+ and FISH+ by central assessment) * BTC: Same as Part 1 * CRC: Same as Part 1 * Tumor measurements as per Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1: * Part 1: Measurable or non-measurable disease * Part 2: Measurable disease * An Eastern Cooperative Oncology Group (ECOG) Performance Status score of 0 or 1 * Adequate organ function * Adequate cardiac left ventricular function, as defined by a LVEF \>/= institutional standard of normal Exclusion: * Prior treatment with a H…