There is this 3rd party tool I have no idea if it (still) works:
http://www.zenphoto.org/news/gallery2-to-zenphoto-migration
I am not familiar with Gallery but if it stores its albums and images in real folders on the file system you could copy these to Zenphoto's /albums folder. However this will not move any data you may have entered via the Gallery backend.
As a former Gallery 2 user, I can confirm that the file structure is similar to that of Zenphoto. There might be a few minor differences, especially with regard with non-pure-ASCII characters (which should be avoided anyways).
So if you own that domain, it should be very easy to move the photos en masse from the Gallery folder to the zenphoto "albums" folder. And because Zenphoto recognizes the structure of the album, you will get the skeleton of your gallery without further intervention.
You don't say :-)
Ye, Gallery3 changed the tables they use a bit. The "gallery2-to-zenphoto-migration" script uses the Gallery2 "FileSystemEntity" table and that table does not seem to exist anymore in Gallery3 and I think this is an important thing to have.
The rest actually seem that it will function fine.
Does anyone else know how to get around this?