Check if WP Gutenberg block exists to load css/js.

functions.php

<?php
add_action( 'enqueue_block_assets', 'example_block_assets');
function example_block_assets() {
  global $post;

  $has_your_own_blocks = false;

  if ( ! is_admin() && is_singular() ) {
    $wp_post = get_post( $post );

    if ( $wp_post instanceof WP_Post ) {
      $has_your_own_blocks = ! empty(
        array_filter(
          array(
            false !== strpos( $wp_post->post_content, '<!-- wp:your-blocks-prefix/' ),
            has_block( 'core/block', $wp_post ),
            has_block( 'core/button', $wp_post ),
            has_block( 'core/cover', $wp_post ),
            has_block( 'core/heading', $wp_post ),
            has_block( 'core/image', $wp_post ),
            has_block( 'core/gallery', $wp_post ),
            has_block( 'core/list', $wp_post ),
            has_block( 'core/paragraph', $wp_post ),
            has_block( 'core/pullquote', $wp_post ),
            has_block( 'core/quote', $wp_post ),
          )
        )
      );
    }
  }

  if ( ! $has_your_own_blocks && ! $this->is_page_gutenberg() ) {
    return;
  }

  // Styles.
  $name       = 'your-own-style-style';
  $filepath   = 'dist/' . $name;
  $asset_file = $this->get_asset_file( $filepath );
  $rtl        = ! is_rtl() ? '' : '-rtl';

  wp_enqueue_style(
    'your-own-plugin-style-name',
    YOUR_PLUGIN_URI . $filepath . '.css',
    array(),
    YOUR_PLUGIN_VERSION
  );
}