Thrice! First put seven gold bars on each side. If they are equal in weight, the one left out is the light one. If not, take the seven on the lighter side and put three on each side of the scale, if they are equal the one kept out will be lighter. If not, take the three on the lighter side and put one on each side and keep the third one out. You will easily realize which one is lighter on the scale, and if the scale is equal, then the one kept out is the lightest.