The next gen plugin probably does not use the post title, but ACF gets the post title to display the post in a relationship field. You would need to figure out what in next gen is the title, most likely some custom field associated with that post type and then use an acf/fields/relationship/result filter to alter what ACF shows.
More than likley, in order to display the gallery you will need construct a sortcode based on the post selected and then call do_shortcode().
