I've not had this problem with GP3edit, but something similar has occurred to me when using GP3 master to export stuff, when it runs out of temporary disk space either on the hard drive where the editor is installed, or on the disk where GP3 was installed. Afterwards the GP3art5.bin file (located in the Bitmaps sub-folder under the GP3 hard disk installation folder) was permanently corrupted, and the only way to fix it and get the game to work properly again was to copy the original GP3art5.bin from the GP3 installation disk to overwrite the corrupt one, and start exporting again from scratch (after freeing up some space on the offending disk
).
Don't know if this is any help to you ...