Set the featured image from the first entry in an ACF gallery field, saving only for a specified post type.

featured image from gallery.php

function set_featured_image_from_gallery() {

    global $post;
    $post_id = $post->ID;

    $has_thumbnail = get_the_post_thumbnail($post_id);

      if ( !$has_thumbnail ) {

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

        if ( $image_id ) {
          set_post_thumbnail( $post_id, $image_id );
        }
      }

}
add_action( 'save_post_property', 'set_featured_image_from_gallery' );

Is it possible to add a code to update a thumbnail after changing order in the "gallery" field?

@lestzx - You can just remove if(!$has_thumbnail ) ie:

function set_featured_image_from_gallery() {

    global $post;
    $post_id = $post->ID;

    $has_thumbnail = get_the_post_thumbnail($post_id);

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

    if ( $image_id ) {
      set_post_thumbnail( $post_id, $image_id );
    }
}
add_action( 'save_post_property', 'set_featured_image_from_gallery' );

Would this work with Custom Post Types?

I have one called 'properties' - Do I need to change anything ?