Hydrolyzes both GDP-mannose and GDP-glucose (PubMed:7592609, PubMed:10913267, PubMed:23481913). Could participate in the regulation of cell wall biosynthesis by influencing the concentration of GDP-mannose or GDP-glucose in the cell. Might also be involved in the biosynthesis of the slime polysaccharide colanic acid (PubMed:7592609, PubMed:10913267). GDP-alpha-D-mannose + H2O = D-mannose + GDP + H(+) Binds 1 Mg(2+) ion per subunit. GDP-alpha-D-mannose is likely to be the biological substrate, but the kcat/KM obtained with GDP-alpha-D-glucose is very similar to that with GDP-alpha-D-mannose (PubMed:7592609). kcat is 0.35 sec(-1) for GDP-mannose (PubMed:23481913). Optimum pH is 9.3. Homodimer. Belongs to the Nudix hydrolase family. Extended N-terminus.