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 265 sec(-1). The catalytic efficiency for GDP-D-glucose is 750-fold higher than for GDP-L-galactose. Expressed throughout the neuronal system, in the spermatheca and anterior hypodermal cells. Expressed in all the normal larval stages observed. 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.