WordPress Gallery Post Thumbnail Editor

I’m having difficulty generating a WordPress gallery from the post thumbnail editor.

I’m using the gallery shortcode do_shortcode('[gallery ids="'.$ids.'" size="medium"]') using a list of ids. I understand that the id list will correspond to the actual images rather than the cropped version. How can I get the id of the cropped version(assuming it’s saved as an image) and if the id doesn’t exist does a way exist to save the cropped version as its own image before loading into the gallery?

Thank You