How to make Attachment Post Type accessible in Gutenberg Editor?

I don’t think it’s how the post type is registered that’s the issue. Attachments are available through the API by using the “media” path. The issue is more in what functions are used to query for lists of posts. Most such functions simply don’t include attachments in their query. Success would depend on whether “attachment” can be added to the query’s post type arg through a filter or not.