Catalyzes the oxidation of L-gulono-1,4-lactone to ascorbic acid (PubMed:20622436). L-gulono-1,4-lactone is oxidized to hydrogen peroxide and L-xylo-hexulonolactone which spontaneously isomerizes to L-ascorbate (By similarity). L-gulono-1,4-lactone + O2 = H(+) + H2O2 + L-ascorbate Cofactor biosynthesis; L-ascorbate biosynthesis. Belongs to the oxygen-dependent FAD-linked oxidoreductase family.