Home directory with MacOs

how can i Set (with My Mac Os) the Default Directory used to save the projects?

The default directory is your macOS user’s home + “Documents/GanttProject”.
In theory, the user home directory can be replaced with some other directory, but technically it is not that easy on macOS because of the tricky launcher application.

Perhaps the easiest way is to create a symbolic link $HOME/Documents/GanttProject which points to the directory you want.

How can I create a symbolic link with Mac?