Signed informed consent; Histologically confirmed gastric, esophageal, or gastro-esophageal junction adenocarcinoma; disease that is locally advanced (unresectable), metastatic, or locally recurrent disease; Measurable or non-measurable, but radiologically evaluable disease, according to RECIST; ErbB2 (HER2)positive; Age =18 years; ECOG Performance status = 2; Adequate organ function, including adequate hematologic, renal and liver function; Cardiac ejection fraction within institutional range of normal as measured by echocardiogram; Able to swallow and retain oral medications, and/or receive enteral medications via gastrectomy feeding tube; Women and men with potential to have children must be willing to practice acceptable methods of birth control during the study; Prior gastric surgery is permitted if > 3 weeks prior and recovered; Prior chemotherapy for non-gastric malignancy if > than 5 years; Prior neoadjuvant and/or adjuvant chemotherapy for early stage gastric cancer if > 6 months since completion; At least 4 weeks since prior radiotherapy; Prior biologic, hormonal, or immunologic cancer treatment if > 5 years since treatment.