Comment 4 for bug 535483

Okay, then it's a bug in GParted and not in parted, I suppose.

Maybe you can also try the following 2 commands in a terminal, in case they show something useful:
LANG=C sudo fdisk -l /dev/sda
LANG=C sudo parted /dev/sda print

(BTW: to do a clean installation you don't need GParted as the installer should be able to do the partitioning too, or if you don't need to change partition sizes/locations, you can just tell it to use the existing partitions--it will erase all files outside /root & /home in that case.)