Here are links that provide step-by-step instructions on how to modify the Gallery2 theme to work with Lightbox in the standalone version, that is, www.yoursite.com/gallery2/index.php, thanks to KayGee.

Ensure Gallery2's image block and image frames plugins are installed and enabled in gallery, lightbox2 module may not pick-up images automatically otherwise because it depends on class names.

The setting "Disable lightbox for gallery lists" in image node settings tab of auto image handling options may affect Gallery2 too.