Set post thumbnail from gallery. Saves a performance hit.

gallery_post_thumbnail.php

function set_featured_image_from_gallery() {
  $has_thumbnail = get_the_post_thumbnail($post->ID);

  if ( !$has_thumbnail ) {

    $images = get_field('gallery', false, false);
    $image_id = $images[0];

    if ( $image_id ) {
      set_post_thumbnail( $post->ID, $image_id );
    }
  }
}

add_action( 'save_post', 'set_featured_image_from_gallery' );