Ah yes, gallery2. Used that quite extensively a few years ago. Have a client with an old gallery2 project as well, we might be doing something with it, so we're probably in the same boat.
I can tell you the last time I looked into the gallery project, they were shutting it down. I have no idea where the project stands now, the original authors wanted someone to take it over if possible. The website is still active but when a project is in limbo, it's never a good sign.
Integrating it with Craft is going to be an exercise in futility and it's going to be just as easy to get the original photos working inside Craft "natively". Craft can really do everything Gallery can do... and better because now you can do things like relate photos back to actual content entries and more.
I took a quick glance at a lot of the site's galleries and it looks like they're not really doing anything special, hardly using any of the gallery features (like tags, etc).
Gallery stores all of its photos in a folder called
g2_data. You could grab that folder, throw it up in a Craft asset source, have Craft index that folder, and everything will be neatly organized by folder ready to roll. Make some Entries that correspond to albums and sub albums and then go to town adding them to each entry/album. Telling Craft about which photos live in which album is going to be the most time consuming but since Gallery 2 stores them hierarchically, at least you have a visual reference vs something simply stored in a flat database structure.
As far as maintainability, Craft is the easiest CMS I've ever had the privilege of using. A lot of my clients figure out most of the control panel before we even have the training session. I remember Gallery2 being very finicky about clearing caches, generating thumbnails, etc. All of this will be handled mostly automatically by Craft.