I am sorry that it happened and glad that you had a kind of backup. However, let’s try to find out what exactly happened, because it reads as if GanttProject formatted your disk
GanttProject corrupted my project files
It was one file, no?
But auto-save was not working.
First, there is no built-in auto-save, in the sense that GanttProject does not automatically write to the file which is opened. So if you don’t save your work for a few hours and it crashes then yes, you may lose what has been done in this period. I am not sure if that was your case though.
Although there is no auto-save , there are files which are written on disk on every undoable action. They are used for undo-redo, however they can be used for recovery too.