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(+) kcat is 26 sec(-1) at 31 degrees Celsius. The catalytic efficiency for GDP-D-glucose is 35-fold higher than for GDP-L-galactose. The orthologs in A.thaliana are GDP-L-galactose phosphorylases catalyzing the first reaction of the Smirnoff-Wheeler pathway, the major route to ascorbate biosynthesis in plants. Belongs to the GDPGP1 family.