Or try this.
MCDU Menu > AIDS
AIDS > ALPHA PARAMETERS
Type in GWFK for Metric or GWFP for imperial into an available line.
Read out the GW as computed by the FACs. As the FACs compute GD from the angle of attack once airborne, this is essentially the same as what has been described earlier, but more accurate. But I'm not sure all customer options allow access to the AIDS in flight, so It may not work on your aircraft.