Gigapascals to millimeters of mercury conversion calculator above calculates how many millimeters of mercury are in 'X' gigapascals (where 'X' is the number of gigapascals to convert to millimeters of mercury). In order to convert a value from gigapascals to millimeters of mercury (from GPa to mmHg) simply type the number of GPa to be converted to mmHg and then click on the 'convert' button.