Oxidizes L-gulono-1,4-lactone to hydrogen peroxide and L-xylo-hexulonolactone which spontaneously isomerizes to L-ascorbate. L-gulono-1,4-lactone + O2 = H(+) + H2O2 + L-ascorbate Cofactor biosynthesis; L-ascorbate biosynthesis via UDP-alpha-D-glucuronate pathway; L-ascorbate from UDP-alpha-D-glucuronate: step 4/4. Highly expressed in liver. Belongs to the oxygen-dependent FAD-linked oxidoreductase family.