Catalyzes a reversible epimerization of GDP-D-mannose that precedes the committed step in the biosynthesis of vitamin C (L-ascorbate), resulting in the hydrolysis of the highly energetic glycosyl-pyrophosphoryl linkage. Able to catalyze 2 distinct epimerization reactions and can release both GDP-L-galactose and GDP-L-gulose from GDP-mannose. GDP-alpha-D-mannose = GDP-beta-L-gulose GDP-beta-L-gulose = GDP-beta-L-galactose Inhibited by GDP and GDP-D-glucose. Cofactor biosynthesis; L-ascorbate biosynthesis via GDP-alpha-D-mannose pathway; L-ascorbate from GDP-alpha-D-mannose: step 1/5. Homodimer. Interacts with chaperone Hsc70-3 protein, which may regulate epimerase activity. Belongs to the NAD(P)-dependent epimerase/dehydratase family.