Gemcitabine Plus Pemetrexed Disodium in Treating Patients With Unresectable or Metastatic Biliary Tract or Gallbladder Cancer

* One of the following histologically or cytologically confirmed cancers not amenable to treatment with combined chemotherapy and radiotherapy: * Biliary tract (intrahepatic, extrahepatic, or ampulla of Vater) carcinoma * Gallbladder carcinoma * Unresectable or metastatic disease * No CNS metastases * Prior brain metastases treated with surgery or radiosurgery allowed provided treatment was completed at least 4 weeks ago and there is no evidence of CNS progression * No clinically significant pericardial or pleural effusion or ascites unless able to be drained before study entry * Performance status - ECOG 0-2 * At least 3 months * Absolute neutrophil count at least 1,500/mm\^3 * Platelet count at least 100,000/mm\^3 * Bilirubin no greater than 3 times upper limit of normal (ULN) * AST no greater than 5 times ULN * Creatinine no greater than 1.5 times ULN * Creatinine clearance at least 45 mL/min * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception * No other malignancy within the past 5 years except adequately treated basal cell or squamous cell skin cancer * Able to tolerate folic acid, corticosteroids, or cyanocobalamin supplements * More than 4 weeks since prior biologic or immunologic therapy * No prior biologic or immunologic therapy for metastatic disease * No concurrent immunotherapy * No concurrent colony-stimulating factors during course 1 * No prior chemotherapy for metastatic disease * No prior gemcitabine * Prior chemoembolization allowed provided the following are true: * At least 4 weeks since prior chemoembolization * Evidence of new tumor growth since therapy * At least 6 months since prior chemotherapy used as a radiosensitizer (in adjuvant setting or for locally advanced disease) * No other concurrent chemotherapy * Prior radiofrequency ablation allowed provided the following are true: * At least 4 weeks since prior radiofrequency ablation * Evidence of new tumor growth since therapy * No prior radiotherapy to 25% or more of the bone marrow * More than 4 weeks since prior radiotherapy * No concurrent radiotherapy * Prior embolization allowed provided the following are true: * At least 4 weeks since prior embolization * Evidence of new tumor growth since therapy * No prior pemetrexed disodium * No aspirin or nonsteroidal anti-inflammatory drugs for at least 2 days (5 days for long-acting agents \[e.g., piroxicam\]) before, during, and for at least 2 days after administration of pemetrexed disodium * No concurrent cyclo-oxygenase-2 inhibitors