ezcontent-8.x-dev/modules/ezcontent_paragraphs/modules/ezcontent_gallery/ezcontent_gallery.module

<?php

/**
 * @file
 * Module file for ezcontent_gallery.
 */

/**
 * Implements hook_preprocess_HOOK().
 */
function ezcontent_gallery_preprocess_paragraph__gallery(&$variables) {
  $variables['#attached']['library'][] = 'ezcontent_gallery/ezcontent_gallery_css';
}