Getting Started

 A standard installation profile is recommend to check Gutenberg Starter theme.

After enabling Gutenberg module and Gutenberg Starter theme (and set it to default), go to the Article content type edit form at /admin/structure/types/manage/article and enable the Gutenberg Experience.

To check field mapping feature, add the following JSON data into the Template field:

[
  ["drupal/simple-text", {"placeholder": "Insert title","tag":"h1", "className": "is-article-title", "mappingFields": [{"field": "title"}]}],
  ["core/image", {"mappingFields":[
    {"field": "field_image", "property":"target_id", "attribute":"id"},
    {"field": "field_image", "property":"alt", "attribute":"alt"}
  ], "className": "is-article-featured-image", "align": "full", "sizeSlug":"full"}],
  ["drupal/section", {"placeholder": "Insert content", "className": "is-article-content"}]
]

And set the Template lock to All.

It should look like this:

Also enable Gutenberg experience for Basic Page content type at /admin/structure/types/manage/page. No need to set any template.