PRIOR CONCURRENT THERAPY: Biologic therapy: No other concurrent biologic therapy Chemotherapy: No prior gemcitabine No prior fluorouracil other than adjuvant fluorouracil received at least 1 year prior to study No other concurrent chemotherapy Endocrine therapy: Concurrent corticosteroids not allowed (except for nausea or vomiting) Radiotherapy: At least 14 days since prior radiation to axial skeleton No concurrent radiotherapy (except to local lesions) Surgery: At least 14 days since prior exploration and biopsy At least 21 days since prior resection with anastomosis No major organ allograft Other: No concurrent barbiturates No concurrent oral anticoagulants No other concurrent investigational drugs or devices