Involved in the biosynthesis of ascorbic acid. Uses L-galactono-1,4-lactone as substrate, but not L-gulono-1,4-lactone, D-galactono-1,4-lactone, D-gulono-1,4-lactone, D-erythronic-1,4-lactone, D-xylonic-1,4-lactone, L-mannono-1,4-lactone, D-galactonic acid, D-glucuronic acid or D-gluconic acid. FAD, NAD, NADP and O(2) cannot act as electron acceptor. 4 Fe(III)-[cytochrome c] + L-galactono-1,4-lactone = 4 Fe(II)-[cytochrome c] + 5 H(+) + L-dehydroascorbate Inhibited by sulfhydryl-modifying agents such as N-ethylmaleimide, monoiodoacetic acid and p-hydroxymercuribenzoic acid. No inhibition by riboflavin and lycorine. Optimum pH is 8.0-8.5. Cofactor biosynthesis; L-ascorbate biosynthesis. The FAD-binding PCMH-type domain does not bind FAD covalently.