lockinInsertCpt.php

function gutenberg_test_cpt_locking() {
	$template = array(
		array( 'core/image' ),
		array(
			'core/paragraph',
			array(
				'placeholder' => 'Add a description',
			),
		),
		array( 'core/quote' ),
	);
	register_post_type(
		'locked-insert-post',
		array(
			'public'        => true,
			'label'         => 'Locked Insert Post',
			'show_in_rest'  => true,
			'template'      => $template,
			'template_lock' => 'insert',
		)
	);
}

add_action( 'init', 'gutenberg_test_cpt_locking' );