In your draft document go to File Properties (Application Button-Properties-File Properties). Select the Units Tab and then select the Advanced Property button. You will see Mass in the left column under Name. Click Kg in the value column and you will see a pulldown arrow where you can adjust the units to Grams. You can also adjust the precision as required.
After setting as desired, click Ok twice to return to the drawing sheet. Click Update All in the Property Text Tab and your mass should update accordingly in the parts list.