I’m a big fan of CMB2 but since Gutenberg (the new WordPress editor) is no longer support the old school PHP meta boxes and highly encourage porting PHP meta boxes to blocks then I looked for an alternative to CMB2 that is compatible with Gutenberg and this what I found:
Gutenberg Compatible Custom Meta Box Framework
It’s not only support adding custom fields that shown under Gutenberg but you can also use ACF to create a Gutenberg Block !
ACF has a free and a paid version.
2. Meta Box
Meta box is also a very good Gutenberg compatible CMB framework.
It is free and has some paid extensions.
It’s also a free choice and I like It’s simplicity and how it’s code organized like WordPress Customization API, so If you are familiar with WordPress Customizer then you will love this framework.
Completely free and powerful custom fields library optimized for developer happiness.
Also creating rich and flexible Gutenberg blocks with Carbon Field is a child’s play.