Unless I misunderstand your question, you want {$new_status}_{my-custom-post-type}
Take a look at the hook registration.
This page (from Pippin’s Posts)* does a better job than I could at explaining it, but from your example, you would want add_action('publish_episode-gallery', 'update_custom_ngg_table');
According to the source, the following status are available:
publish, future, draft, pending, private, trash, auto-draft and inherit.