To work with Colorbox, this
<?php
$image_ids = get_field('gallery', false, false);
$shortcode = '
[gallery ids="' . implode(',', $image_ids) . '"]
';
echo do_shortcode( $shortcode );
?>
needs to be
<?php
$image_ids = get_field('gallery', false, false);
$shortcode = '
[gallery ids="' . implode(',', $image_ids) . '", link="file"]
';
echo do_shortcode( $shortcode );
?>