Block Template for a Page Template in Gutenberg

block_template_by_page_template.php

<?php
function pls_maybe_add_template() {
	if ( ! is_admin() || ! isset( $_GET['post'] ) || 'template-bridge.php' !== get_page_template_slug($_GET['post']) ) {
		// This is not the post/page we want to limit things to.
		return false;
	}

	$post_type_object = get_post_type_object( 'page' );
	$post_type_object->template = array(
			array( 'core/heading', array(
					'placeholder' => 'Add Description...',
			) ),
	);
	$post_type_object->template_lock = 'all';
}
add_action( 'init', 'pls_maybe_add_template' );