removes the slug from a custom post type

remove_custom_post_type_slug.php

/**
 * credit to: https://wordpress.stackexchange.com/q/291735
 *
 */

/**
 * remove the slug from published gallery permalinks. 
 * 
 * Makes sure to only touch our CPT
 * 
 */
function we_remove_gallery_cpt_slug( $post_link, $post ) {

  // check that we're only working with gallery-new posts, and that they're published
  if ( 'gallery-new' === $post->post_type && 'publish' === $post->post_status ) {

    // replace 
    $post_link = str_replace( '/' . $post->post_type . '/', '/', $post_link );
  }

  return $post_link;

}
add_filter( 'post_type_link', 'we_remove_gallery_cpt_slug', 10, 2 );

function we_add_gallery_cpt_posts_to_main_query( $query ) {

  if ( ! $query->is_main_query() || 2 != count( $query->query ) || ! isset( $query->query['page'] ) ) {
    return;
  }

  if ( ! empty( $query->query['name'] ) ) {
      $query->set( 'post_type', array( 'post', 'gallery-new', 'page' ) );
  } elseif ( ! empty( $query->query['pagename'] ) && false === strpos( $query->query['pagename'], '/' ) ) {
      $query->set( 'post_type', array( 'post', 'gallery-new', 'page' ) );

      // We also need to set the name query var since redirect_guess_404_permalink() relies on it.
      $query->set( 'name', $query->query['pagename'] );
  }

}
add_action( 'pre_get_posts', 'we_add_gallery_cpt_posts_to_main_query');