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 HER2-targeted agent

          -  Prior systemic anti-cancer therapy (including investigational products) except prior
             adjuvant/neoadjuvant therapy, which must be completed at least 6 months prior to first
             study treatment dosing. For subjects with BTC and CRC the following additional
             exceptions apply:

               -  BTC: patients may have started therapy for advanced disease but may not have
                  received more than one cycle of any standard gemcitabine-based chemotherapy
                  regimen.

               -  CRC: patients may have started therapy for advanced disease but may not have
                  received more than one cycle of 5-FU-based chemotherapy (< 1 month of therapy).

          -  Patients with certain contraindications to bevacizumab cannot be enrolled on the
             mFOLFOX6-2 with bevacizumab arm.

          -  Palliative radiotherapy is allowed if completed at least 2 weeks prior to first study
             treatment dosing

          -  Untreated known brain metastases (patients with treated brain metastases who are off
             steroids, off antiseizure medications, and stable for at least 1 month at the time of
             screening are eligible)

          -  Clinically significant cardiac disease, such as ventricular arrhythmia requiring
             therapy, uncontrolled hypertension or any history of symptomatic congestive heart
             failure (CHF). Patients with known myocardial infarction or unstable angina within 6
             months prior to randomization are also excluded.

          -  QTc Fridericia (QTcF) > 470 ms. For patients with longer QTcF on initial
             electrocardiogram (ECG), follow-up ECG may be performed in triplicate to determine
             eligibility

          -  Peripheral neuropathy > Grade 1 per NCI-CTCAE v5.0

          -  Clinically significant interstitial lung disease

          -  Prior or concurrent malignancy whose natural history or treatment has the potential to
             interfere with the safety or efficacy assessment of the investigational regimen

          -  Active hepatitis B or hepatitis C infection or infection with Human Immunodeficiency
             Virus (HIV)-1 or HIV-2 (Exception: patients with well controlled HIV [e.g., CD4 >
             350/mm3 and undetectable viral load] are eligible)