I have finished all (Gallery3) Menalto to Zenphoto Migration PHP Scripts.
https://github.com/mebels/men-to-zen-migration/tree/master
I could never get this to work as it seems I had a lot of non-standard characters in my Gallery3 titles. I now have 44,000 photos I can't move across!
The script is pretty old, not maintained for years and therefore not tested with current Zenphoto installs…
Zenphoto is file system based so you can move images and albums via FTP. If meta data is embeded into the images that will be read by Zenphoto generally.
I believe @RichardB has recently moved Gallery sites to Zenphoto and may be of help here.
I gave up as I have a set limit on the web server for space and did not want to break anything further. Also having to do mod rewrites on thousands of pages.