Catalytic subunit of the GMPPA-GMPPB mannose-1-phosphate guanylyltransferase complex (PubMed:33986552). Catalyzes the formation of GDP-mannose, an essential precursor of glycan moieties of glycoproteins and glycolipids (PubMed:33986552). Can catalyze the reverse reaction in vitro (PubMed:33986552). Together with GMPPA regulates GDP-alpha-D-mannose levels (PubMed:33986552). . This is the function of Ensembl gene identifier ENSG00000173540 (GMPPB, GDP-mannose pyrophosphorylase B).