Gutenberg Custom Fields
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.
Documentation
Documentation can be found here. If you don't find the answers you're looking for, submit an issue.