This is, at least at the moment, not possible. You can use CMB2 Meta Boxes in Gutenberg Post Types, but you can not make a CMB2 Field where you can use Gutenberg – as of now.
However, as the Gutenberg Editor is developed further, a CMB2 Block-Editor-Field is not that far. As of now, you can already integrate the Block Editor on your own admin page (see https://github.com/getdave/standalone-block-editor for instance).
So, if you only need a settings page with a Block Editor, you could use this. You could also try and implement this code into a custom CMB2 Field (I’m not sure this is possible at the moment).
Happy Coding!