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(+) 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. By jasmonate, ozone and high light. Circadian-regulation, with a peak in expression at the beginning of the light cycle. No visible phenotype and slightly decreased ascorbate levels; due to the partial redundancy with VTC2. Vtc2 and vtc5 double mutants show growth arrest immediately upon germination and are not viable. Belongs to the GDPGP1 family.