Catalyzes a reaction of the Smirnoff-Wheeler pathway, the major route to ascorbate biosynthesis in plants. Acts as a phosphorylase rather than as a transferase. Uses preferentially GDP-L-galactose and GDP-D-glucose as substrates. Lower activity with GDP-L-fucose, very low activity with GDP-D-mannose, and no activity with UDP-D-glucose, UDP-D-galactose or ADP-D-glucose. Highly specific for inorganic phosphate as the guanylyl acceptor. GDP-beta-L-galactose + phosphate = beta-L-galactose 1-phosphate + GDP + H(+) Not inhibited by dithiothreitol, N-ethylmaleimide, phenylmethane sulfonyl fluoride, ascorbate, L-galactose and L-galactonolactone. Optimum pH is 7.5. Cofactor biosynthesis; L-ascorbate biosynthesis via GDP-alpha-D-mannose pathway; L-ascorbate from GDP-alpha-D-mannose: step 2/5. Interacts with TLP1. Expressed in leaves, stems, roots, flowers and siliques. Highest expression in green tissues. Expressed in all developmental stages. By jasmonate, ozone and high light. Circadian-regulation, with a peak in expression at the beginning of the light cycle. Dwarf. 20% of the wild-type ascorbate level, due to the partial redundancy with VTC5. Vtc2 and vtc5 double mutants show growth arrest immediately upon germination and are not viable. Belongs to the GDPGP1 family. According to publications, it is related to the galactose-1-phosphate uridylyltransferase type 1 family and histidine triad superfamily (PubMed:12119013). However, such families are not detected by prediction tools such as Pfam or SUPFAM.