The first is that it's a completely interactive GIMP plug-in. It will do a non-interactive batch mode, too, but that's nothing special, really. When in interactive mode, however, it loads each image in the directory in sequence and lets you look at the image, decide if you want to include it in the directory or not, allow you to give it a title and caption, and perform any adjuments needed (rotation, cropping, adjusting contrast, etc) from within GIMP. Any changes you make are unsaved (unless you manually select "File->Save") so your originals stay clean. When you're done, Gimp Gallery automagically scales your picture as designated during the setup, and adds it to the gallery.
The second feature that makes Gimp Gallery different is that if you don't like default gallery template (there is current only one template but I'd like to change that), you can create your own by writing a custom Perl function. This function accepts a GalleryInfo object and from there can gather all the information it needs to create its own HTML for the gallery. Most other systems use XML files or something else to create the template, which may or may not be more to your liking. I dunno.
Here is a screenshot of the template options.
Here is a screenshot of the program in the process of creating a gallery. (Here's a smaller version)
Here is a screenshot of the custom gallery options.
Here is the ChangeLog