Inclusion Criteria:

          -  Adult males or females (aged 18 or older) with histologically confirmed, metastatic
             human epidermal growth factor receptor 2 (HER2)+ (by immunohistochemistry (IHC) 3+ or
             fluorescence in situ hydridization (FISH) ratio ≥ 2.0) breast cancer

          -  Have progressed on at least one prior line of chemotherapy plus HER2 directed therapy
             such as trastuzumab and/or pertuzumab in the metastatic setting. T-DM1 would count as
             a line of therapy and patients previously treated with T-DM1 are eligible.

          -  Have not been treated with gemcitabine in the metastatic setting

          -  Measurable disease per Response Evaluation in Solid Tumors (RECIST) 1.1 criteria

          -  Eastern Cooperative Oncology Group (ECOG) performance status 2≤

          -  Left Ventricular Ejection Fraction (LVEF) ≥ 50% at baseline as determined by either
             echocardiogram (ECHO) or multiple gated acquisition scan (MUGA)

          -  Adequate bone marrow function as indicated by the following: absolute neutrophil count
             (ANC) >1500/µL; Platelets ≥100,000/µL; Hemoglobin >10 g/dL

          -  Adequate renal function, as indicated by creatinine ≤1.5x upper limit of normal (ULN)

          -  Adequate liver function, as indicated by bilirubin ≤1.5x ULN, aspartic transaminase
             (AST) or alanine transaminase (ALT) <2x ULN unless related to metastatic breast cancer
             to the liver (in which case AST/ALT < 5x ULN is allowed).

          -  Signed informed consent

          -  Adequate birth control in sexually active women of childbearing potential

        Exclusion Criteria:

          -  Active uncontrolled infection or major concurrent illness which in the opinion of the
             investigator would render the participant unsafe to proceed with the study

          -  Uncontrolled central nervous system (CNS) metastases. Treated, non-progressing CNS
             disease (documented by brain magnetic resonance imaging [MRI]) off corticosteroids for
             at least 1 month potential participants are eligible.

          -  Women who are pregnant or lactating

          -  Prior chemotherapy within the last 3 weeks (last 6 weeks for nitrosureas/mitomycin)

          -  Prior radiation therapy within the last 4 weeks; prior radiation therapy to indicator
             lesion (unless objective disease recurrence or progression within the radiation portal
             has been documented since completion of radiation)

          -  Other concomitant active malignancies

          -  History of significant cardiac disease, cardiac risk factors or uncontrolled
             arrhythmias

          -  Ejection fraction <50% or below the lower limit of the institutional normal range,
             whichever is lower

          -  Hypersensitivity to any of the study medications

          -  Untreated psychiatric conditions preventing informed consent