Modify post type for Gutenberg

functions.php

function modify_post_type() {
 	
	//new args
	$args = array(
		'public' => true,
		'label'  => 'Posts',
		'show_in_rest' => true,
		'template' => array(
			array( 'core/cover-image' ),
			array( 'core/image' ),
			array( 'core/quote' ),
			array( 'core/heading' ),
			array( 'core/embed' ),
			array( 'core/paragraph' ),
			array( 'core/paragraph' ),
			array( 'core/paragraph' ),
		),
	);

	//re-register the post type to include new args
    register_post_type( 'post', $args );
}
add_action( 'init', 'modify_post_type' );

You can also do:

$post_type = get_post_type_object( 'post' );
$post_type->template = array(
  array( 'core/cover-image' ),
  array( 'core/image' ),
  array( 'core/quote' ),
  array( 'core/paragraph' ),
);

Thanks!