Is it possible to delete the whole library at once?
I tried removing the paths in preference and then start control titles, but that will take several hours and i'm not even sure that will do the job.
It would be much easier if there was a button "Clean library".
Or is there a file that i can delete, where all data is stored?
Please don't accept my reply as the authoritative solution as I am new to GMB. But I think if you remove (or rename) the following file it will completely clear the library and all your settings, making GMB like a fresh installation:
You can also edit the file with a text editor.