Involved in GDP-mannose biosynthesis which serves as the activated sugar nucleotide precursor for mannose residues in cell surface polysaccharides. This enzyme participates in synthesis of the LPS group B O antigen. alpha-D-mannose 1-phosphate + GTP + H(+) = diphosphate + GDP-alpha-D-mannose Nucleotide-sugar biosynthesis; GDP-alpha-D-mannose biosynthesis; GDP-alpha-D-mannose from alpha-D-mannose 1-phosphate (GTP route): step 1/1. Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Homodimer. Belongs to the mannose-6-phosphate isomerase type 2 family.