Catalyzes the oxidation of guanosine diphospho-D-mannose (GDP-D-mannose) to GDP-D-mannuronic acid, a precursor for alginate polymerization. The alginate layer causes a mucoid phenotype and provides a protective barrier against host immune defenses and antibiotics (By similarity). GDP-alpha-D-mannose + H2O + 2 NAD(+) = GDP-alpha-D-mannuronate + 3 H(+) + 2 NADH Glycan biosynthesis; alginate biosynthesis. Belongs to the UDP-glucose/GDP-mannose dehydrogenase family.