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
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
Can you manage?