Sometimes, we refer to these as "page templates". The idea is to be able to edit a large content area within Gutenberg and save things in and outside post content.

If we consider the current Gutenberg editor as an editor bootstrapped with the following template (A template containing two blocks a post title block and a post content block)

[ { name: 'core/post-title' }, { name: 'core/post-content' } ]

From that point, Gutenberg should be able to be bootstrapped with any block template,. The blocks inside the post-content block would be saved into the post content (like today) and the blocks outside this area constitues a template object, that could potentially be saved to a template CPT.