The purpose of this study is to see if a combination of drugs can help to treat breast cancer. This is a Phase II study which will test whether this combination of drugs is effective and will provide further information on side effects and safety. A standard chemotherapy, gemcitabine, will be combined with 2 other drugs that target the HER2 receptor. The HER2 receptor is a growth protein on the surface of some breast cancer cells that provides messages telling the breast cancer cell to grow. The standard chemotherapy will be gemcitabine.. The other two drugs work against HER2. One is called trastuzumab (Herceptin) and it is commonly given to women with advanced and early HER2 positive breast cancer. The other drug, pertuzumab (Perjeta), is also given to women with advanced HER2 positive breast cancer. The drugs in this study are each individually approved for the treatment of metastatic breast cancer. However, this study is looking at the effectiveness of gemcitabine with trastuzumab and pertuzumab when given to women who have advanced HER2 positive breast cancer who have had prior trastuzumab + pertuzumab or pertuzumab-based therapy.