I want to layout post content with Gutenberg because I'm designing the Post Singular layout for the custom post type with Beaver Themer. The block editor gives me more layout options than the classic editor.
When editing the post - all post types - the default editor is Block Editor (Gutenberg). Have you activated the plugin Classic Editor? This plugin also allows the user to switch the editor by going to Settings -> Writing. See my screenshot https://share.getcloudapp.com/qGuK8yLY.