Gigajoules to gigacalories conversion calculator shown above calculates how many gigacalories are in 'X' gigajoules (where 'X' is the number of gigajoules to convert to gigacalories). In order to convert a value from gigajoules to gigacalories (from GJ to Gcal) type the number of GJ to be converted to Gcal and then click on the 'convert' button.