Wordpress / ACF : Set gallery image from featured image. If featured image is set and gallery field is empty.

functions.php

add_action( 'save_post', 'set_first_gallery_image_from_featured_image' );

function set_first_gallery_image_from_featured_image() {

  $post = get_post();
  $featured_image = get_the_post_thumbnail($post->ID);
  $acf_gallery = get_field('images', false, false); // Change images to the name of the ACF gallery field you want to update
  $attachments_ids = [ 0 => get_post_thumbnail_id($post->ID) ];

    if ( $featured_image && !$acf_gallery ) {
        update_field('images', get_post_thumbnail_id($post->ID), $post->ID);
        //delete_post_thumbnail($post->ID); // Uncomment to also delete the post thumbnail
    }

}