Produces a precursor for alginate polymerization. The alginate layer provides a protective barrier against host immune defenses and antibiotics. D-mannose 6-phosphate = D-fructose 6-phosphate alpha-D-mannose 1-phosphate + GTP + H(+) = diphosphate + GDP-alpha-D-mannose Co(2+) (for PMI). Nucleotide-sugar biosynthesis; GDP-alpha-D-mannose biosynthesis; GDP-alpha-D-mannose from alpha-D-mannose 1-phosphate (GTP route): step 1/1. Nucleotide-sugar biosynthesis; GDP-alpha-D-mannose biosynthesis; alpha-D-mannose 1-phosphate from D-fructose 6-phosphate: step 1/2. Monomer. Belongs to the mannose-6-phosphate isomerase type 2 family.