Gutenberg Custom Fields allows you to control the content of the Gutenberg edit screen by creating pre-filled templates.
Navigate to the “GCF” admin page, create a new template, select a post type and add fields as you wish.
The Gutenberg Editor will be pre-filled with the corresponding post type’s template.
This plugin is being worked on in GitHub
Documentation can be found here. If you don’t find the answers you’re looking for, submit an issue.