Hi Jan,
I don’t know…
There was some kind of Widget Block, but it is gone or never even made it into a release.
I think WordPress Core doesn’t see classic widgets as an option for blocks.
You could still use the shortcode for reading, like [gwolle_gb_read].
Now that I look into it, there is a shortcode for the widget that I forgot all about :)
[gwolle_gb_widget] is the shortcode.
Options are for book_id, num_entries and num_words.
Can you manage?