Recently I migrated my photo album from Gallery2 to Zenphoto. It wasn't an easy process because there is no good out-of-the-box tool written for the migration. So, I mashed together some bits and pieces that others had published online, added some of my own perl glue, and came up with the following Frankenstein script that did 85-90% of the work for me. Because I'd switched from putting descriptions in the summary field to the title field at some point during my run with Gallery2, I had to hand-import some information. In addition, if you've used any of Gallery2's tags (to add a link to a field or bold some text or something) this script won't fix it. You can probably pretty easily fix all those problems with a few regexes in strategic places, but I only had a few in my own installation so it wasn't worth the time to build that into the script.
There are a couple of gotchas involved with the script, especially if you have multiple albums in Gallery2 that all have the same name or if you're importing something beyond JPEGS and albums. Reading through the code is definitely worth your time.
When I ran the script, it was against version 2.3.something of Gallery2 and 1.4.3 of zenphoto.
In the spirit of open source and sharing, the script I used can be downloaded from github. I hope that someone else finds it useful.