1. take an external usb disk containing an initial fat32 partition
2. tell gparted to delete the partition and create a new fat32 one in its place
3. press the apply button

When you do this, gparted applies the actions.  Right after creating a new fat32 partition, Gnome Volume Manager detects it and mounts it (because it contains data from the old file system that was nor erased).  gparted then calls mkfs which fails because the device is already mounted.

I think it would a good idea to send a DBUS message to Gnome Volume Manager asking it to ignore all events while gparted is working...