Custom fields are already supported and if you have a custom post type with meta fields, which is enabled for the rest api, you can install gutenberg and see how it looks. If it’s the final layout, I’m not sure but it does seem to work for the most part.
I have only encountered a few bugs relating to fields utilizing a color picker or date picker.
You can find more info here https://www.ads-software.com/gutenberg/handbook/reference/meta-box/