Phase 3 Study to Evaluate the Efficacy and Safety of HER2/Neu Peptide GLSI-100 (GP2 + GM-CSF) in HER2/Neu Positive Subjects

Inclusion Criteria: * Histologically confirmed diagnosis of HER2/neu positive primary breast cancer for all tumors biopsied at diagnosis (multifocal, multicentric, or synchronous contralateral disease) * Completion of both neoadjuvant and adjuvant trastuzumab-based standard of care breast cancer therapy * Stage I, II, or III at presentation with pathologic evidence of residual invasive carcinoma in the breast or axillary lymph nodes (residual disease) at surgery following completion of neoadjuvant therapy -OR- Stage III at presentation with pathologic complete response (pCR) at surgery following completion of neoadjuvant therapy * The subject can begin study therapy within one year of completion of adjuvant trastuzumab-based therapy and any other standard therapies, but, study therapy can be administered concurrently with endocrine therapy. * No clinical evidence of residual or persistent breast cancer per treating physician assessment * ECOG 0-2 * Adequate organ function * Negative pregnancy test or evidence of post-menopausal status * If of childbearing potential, willing to use a form of highly effective contraception * Subject must both reside in and have been treated for their cancer in the country in which the clinical site is located. Exclusion Criteria: * Stage IV cancer or metastatic breast cancer at any time * Inflammatory breast cancer * Receiving other investigational agents * Receiving chemotherapy * Requiring long-term systemic treatment with corticosteroids or other immunosuppressive therapy * History of immunodeficiency or active autoimmune disease * A history of serious allergic reactions, including anaphylaxis, to human granulocyte-macrophage colony-stimulating factors such as sargramostim, yeast-derived products, or any component of the investigational product * Other malignancies except adequately treated in situ carcinoma of the cervix or basal cell or squamous cell carcinoma of the skin * Active infection * Known HIV infection with a detectable viral load within 6 months of the anticipated start of treatment. Note: Subjects on effective antiretroviral therapy with an undetectable viral load within 6 months of the anticipated start of treatment are eligible for this trial.