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. No activity with cathechol and protecatechuate as substrates. Part of a 3-hydroxybenzoic acid-degradation pathway. 2,5-dihydroxybenzoate + O2 = 3-maleylpyruvate + H(+) Inhibited by 2,2'-dipyridyl. Highest activity in the presence of 2 M KCl or NaCl. 60% loss of activity in the presence of 1.5 M KCl. Optimum pH is 7.2. Optimum temperature is 45 degrees Celsius. Aromatic compound metabolism; naphthalene degradation. Homotetramer. Up-regulated in cells grown on benzoic acid, 3-hydroxybenzoic acid, phenylpropionic acid or cinnamic acid, but not in cell grown on pyruvic acid. Belongs to the gentisate 1,2-dioxygenase family. Extended N-terminus.