Catalyzes the oxygen-dependent ring fission of gentisate between the carboxyl and proximal hydroxyl groups at positions 1 and 2 of the aromatic ring to form maleylpyruvate. Can also catalyze oxidation of alkyl- and halogenated gentisates. Exhibits higher affinity for 3-substituted gentisates than for gentisate but has higher activity with gentisate. 2,5-dihydroxybenzoate + O2 = 3-maleylpyruvate + H(+) Aromatic compound metabolism; naphthalene degradation. Belongs to the gentisate 1,2-dioxygenase family.