Catalyzes the formation of GDP-mannose, an essential precursor of glycan moieties of glycoproteins and glycolipids. 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. Associates with GMPPA. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the transferase hexapeptide repeat family.