Attachment Custom Class

Hi, firstly this is a fantastic plugin!!

I am creating a new gallery and see that the Custom Class for each attachment automatically renders in the link attribute for each attachment. How would I echo the custom class for each attachment separately so that I can add it to a different part of the gallery html? I have tried various methods such as $custom_class, $attachment->custom_class, but nothing is generated. Any help is greatly appreciated.

Thank you.