Using latest Debian sid, there might be a problem with gparted being run within KDE, failing to load the $DISPLAY.
$ sudo gparted No protocol specified (gpartedbin:3160): Gtk-WARNING **: cannot open display: :0
normally you would then keep the display within sudo session (# visudo and add Defaults env_keep=DISPLAY), but install kdesudo will resolve the actual problem (don’t keep the display with sudo, otherwise kdesudo will fail!).
$ sudo apt-get update ; apt-get install kdesudo $ kdesudo gparted