1 Gm (Gigameter) is equal to 999,999,999,999.9999 mm (Millimeter). To convert from Gigameter to Millimeter, multiply by 1,000,000,000,000 or divide by 0.000000000001.
Nine Hundred Ninety-Nine Billion Nine Hundred Ninety-Nine Million Nine Hundred Ninety-Nine Thousand Nine Hundred Ninety-Nine Point Nine Nine Nine Nine Millimeter