After transfer of sugars to endogenous macromolecular acceptors, the enzyme converts nucleoside diphosphates to nucleoside monophosphates which in turn exit the Golgi lumen in a coupled antiporter reaction, allowing entry of additional nucleotide sugar from the cytosol. GDP + H2O = GMP + H(+) + phosphate Protein modification; protein glycosylation. Homodimer. Present with 8680 molecules/cell in log phase SD medium. Belongs to the GDA1/CD39 NTPase family.