Fatal error: Undefined class name 'galleryembed'

This seems to be caused by enabling a gallery block without correctly configuring the path parameter of the gallery module. Prior to enabling any blocks, go to administer/settings/gallery in the navigation menu, and fill in the "Location of Gallery2" field. The path required seems to be the relative path from the drupal root directory to the gallery2 directory. For me, that's some horrible thing with lots of ../../../ involved, so I just symlinked gallery2 into the drupal directory, which seemed to work.

While you're here, remember to check some of the "Image Block" and "Show" checkboxes, or things won't work.

It's a pretty evil error, though, drupal blows up completely if it hits it. You can set the status of the gallery modules in the mysql blocks table to 0 while you fix it.