Bortezomib in Treating Patients With Unresectable or Metastatic Gastric Cancer or Gastroesophageal Junction Adenocarcinoma

DISEASE CHARACTERISTICS: * Histologically or cytologically confirmed unresectable or metastatic gastric or gastroesophageal junction adenocarcinoma * Siewert's class II or III disease * Measurable disease * At least 1 unidimensionally measurable lesion at least 20 mm by conventional techniques OR at least 10 mm by spiral CT scan * No history of known or active brain metastases or primary brain tumors PATIENT CHARACTERISTICS: Age * 18 and over Performance status * Karnofsky 60-100% OR * ECOG 0-2 Life expectancy * Not specified Hematopoietic * WBC at least 3,000/mm\^3 * Absolute neutrophil count at least 1,000/mm\^3 * Platelet count at least 100,000/mm\^3 Hepatic * Bilirubin no greater than 1.5 mg/dL * AST and ALT no greater than 3 times upper limit of normal (ULN) (5 times ULN if liver metastases are present) Renal * Creatinine no greater than 1.5 mg/dL Cardiovascular * No prior cerebrovascular event * No prior orthostatic hypotension * No myocardial infarction within the past 6 months * No peripheral vascular disease requiring surgical management * No evidence of acute ischemia or significant conduction abnormality by EKG Other * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception during and for at least 2 months after study participation * No evidence of peripheral neuropathy * No prior allergic reaction attributed to compounds of similar chemical or biological composition to bortezomib * No other malignancy within the past 3 years except basal cell skin cancer, carcinoma in situ of the cervix, or adequately treated nonmetastatic prostate cancer * No ongoing or active infection * No psychiatric illness or social situation that would preclude study participation * No other concurrent uncontrolled illness that would preclude study participation * No other medical condition or reason that would preclude study participation PRIOR CONCURRENT THERAPY: Biologic therapy * More than 3 weeks since prior immunotherapy * No concurrent biological or immunological agents Chemotherapy * No more than 1 prior chemotherapy regimen (including adjuvant chemotherapy) * More than 3 weeks since prior chemotherapy (6 weeks for nitrosoureas or mitomycin) * No concurrent chemotherapy Endocrine therapy * Not specified Radiotherapy * More than 3 weeks since prior radiotherapy * No concurrent radiotherapy Surgery * Not specified Other * Recovered from all prior therapy * No other concurrent investigational agents * No other concurrent anticancer agent or therapy