Nucleoside diphosphate sugar hydrolase that hydrolyzes GDP-mannose as its preferred substrate, yielding GMP and mannose-1-phosphate. GDP-alpha-D-mannose + H2O = alpha-D-mannose 1-phosphate + GMP + 2 H(+) Homodimer. In the dimer, the N-terminal domains are swapped between the two monomers, such that residues of both chains contribute to the active site. Belongs to the Nudix hydrolase family. NudK subfamily.