Attach images to posts using custom fields (just paths to images already uploaded)

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()).

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