Example of how to set default blocks for a post in Gutenberg with block templates.

gutenberg-block-tempalte-example.php

<?php

function wpvipblocks_templates( $args, $post_type ) {

  if ( $post_type == 'post' ) {
    $args['template_lock'] = true;
    $args['template'] = [
      [
        'core/heading', [
          'placeholder' => 'Add heading here.',
        ]
      ],
      [
        'core/image', [
          'align' => 'right',
        ]
      ],
      [
        'core/paragraph', [
          'align' => 'left',
          'placeholder' => 'Add your description here.',
        ]
      ]
    ];
  }

  return $args;

}
add_filter( 'register_post_type_args', 'wpvipblocks_templates', 20, 2 );