Hi Elena,
You’re not the first person to suggest that functionality so I may start more seriously considering adding an exclude option in the future, but for now you cannot do what you are asking (not directly, anyway).
You can, however, have multiple document galleries on a single page by using the ids parameter. This parameter works by explicitly listing (comma-separated) all of the attachments you want in the document gallery in the order you want them to appear.
Example:
If I want to have two document galleries, one with attachments 1,5, and 7 then I would put this:
[dg ids=1,5,7]
I can then have a second gallery with 2, 3, and 9:
[dg ids=2,3,9]
You can get the ids for a given attachment in a number of ways, but one way is to go to the edit page for that attachment and pull the number of the attachment out of the URL. From my site, I pulled this partial URL: wp-admin/post.php?post=371&action=edit. The ID you would use to include this attachment in a document gallery is 371.
I hope this helps you!
-Dan
PS: If you’ve found this plugin helpful, a good rating would be much appreciated!
I’m going to mark this topic as resolved, but feel free to post again if you have any additional questions.
-Dan
