Ring-cleavage dioxygenase that acts specifically on gallate to produce the keto-tautomer of 4-oxalomesaconate. Mediates the first step of gallate degradation pathway. 3,4,5-trihydroxybenzoate + O2 = (1E)-4-oxobut-1-ene-1,2,4-tricarboxylate + 2 H(+) Optimum pH is 7.0. By gallate. Belongs to the LigB/MhpB extradiol dioxygenase family.