Yes it is. As noted in the documentation for register_post_type() (emphasis mine):
‘show_in_rest’
(bool) Whether to include the post type in the REST API. Set this to true for the post type to be available in the block editor.
This is because the block editor (Gutenberg) is completely powered by the REST API. If the post type is not accessible via REST then the block editor cannot load or save the post.