Migrate gallery2 site to new Drupal site

Hello,

We have a standalone site running gallery2. We'd like to migrate the contents of this site to our new Drupal site (Drupal 6.22). We have several hundred albums containing several thousand images for a total of about 15GB of images.

How can we go about doing this as painlessly as possible? Can we install gallery3 to our Drupal site right off the bat, or is it better to upgrade once the migration is done?

Any help appreciated!