User guides for PizazzWP blocks and plugins for Headway and WordPress
GalleryPlus User Guide

Adding images to an existing Gallery+ gallery (WordPress 3.4.x or earlier)

To add an image to an existing gallery, all you need to do is go back into the gallery, open it up and click on the add image icon and upload the images.

Remember you don’t insert into the post though.

Do "Save All Changes".

Then Update the gallery.

The only other extra thing you need to do is Refresh Image Cache under the PizazzWP, Tools sub-menu.