The new style creates a somewhat-chaotic arrangement, taking each image's size and aspect ratio into consideration to create a unique layout that's dependent on the gallery's content.
A new parallax setting offers another 'dimension' to this design, as well.
To try it out, simply click the 'edit' button at the top-left of any photo gallery section, then the 'Layout' tab, and finally, select the 'Collage' layout.