Hope this helps you
// add filter w/high priority so it runs after ACF
add_filter('acf/format_value/name=gallery', 'add_values_to_gallery', 20, 3);
function add_values_to_gallery($value, $post_id, $field) {
if (!$value || !is_array($value) || !count($value)) {
// no value, bail early
return $value;
}
// loop through gallery
// note & before $images => pass by reference
// so that changes to $image effect original
// http://php.net/manual/en/language.references.pass.php
foreach ($value as $index => &$image) {
// add a value to this image
$image['my-new-index'] = 'my new value';
}
return $value;
}
