I would recommend just modifying the single-listings.php
template file to output the gallery directly:
<?php echo do_shortcode( ' ); ?>
That way, for all the images that have been imported and attached to a given CPT, they weill be output just as if you had placed a shortcode in the post_content for each post.
Edit
If you have post metadata that contains image URL paths, and you want to upload/attach those images to the post, then have a look at media_sideload_image()
(or, in 3.5, wp_sideload_image()
).