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. It is not yet known whether gemcitabine and erlotinib are more effective when given alone or together in treating non-small cell lung cancer.