Specific and highly efficient GDP-D-glucose phosphorylase regulating the levels of GDP-D-glucose in cells. GDP-alpha-D-glucose + phosphate = alpha-D-glucose 1-phosphate + GDP + H(+) Belongs to the GDPGP1 family. The ortholog in A.thaliana catalyzes the first reaction of the Smirnoff-Wheeler pathway, the major route to ascorbate biosynthesis in plants.