Involved in cell wall synthesis where it is required for glycosylation. Involved in cell cycle progression through cell-size checkpoint. 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. Belongs to the transferase hexapeptide repeat family.