<qa>  What is gemcitabine used in chemotherapy?RATIONALE: Drugs used in chemotherapy, such as gemcitabine, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Erlotinib may stop the growth of tumor cells by blocking some of the enzymes needed for cell growth. Giving gemcitabine together with erlotinib may kill more tumor cells. 