Hello Alfred,
NOTE: It is important that you perform a backup with GTB as soon as possible!
How long since the tasks were deleted? When a task is deleted, the 'deleted' flag is set, and the task is no longer displayed. At some indeterminate time in the future (could be hours, days or possibly weeks), the Google Tasks server deletes tasks that have the 'deleted' flag set, and then they are lost forever.
Google Tasks Backup can retrieve tasks that have been marked as deleted. Be sure to enable all 3 options (Include completed/deleted/hidden tasks) before clicking the [Retrieve tasks] button on the GTB main page.
The retrieved tasks can then be viewed, or exported by GTB in a number of different formats.
If your intention is to re-import the deleted tasks, then the "Import/Export CSV" format is most suitable.
I would recommend also exporting as "Import/Export GTBak". This format cannot be edited, but it is much better at handling international and extended characters.
You can use a spreadsheet program to filter only the deleted tasks in the CSV file. Copy the deleted tasks to a new CSV file, and clear the deleted flag, then import that CSV file using
Google Tasks Import.
Let me know if you need more details.
I hope this helps,
Julie Smith
GTB/GTI Developer