Gallery2 is missing from Debian Lenny (currently in testing, but about to become stable). I'm doing a migration this weekend, so I had to grab the package from the sid (unstable) repository. Here's what I did.
Package: * Pin: release a=unstable Pin-Priority: 200 Package: gallery2 Pin: release a=unstable Pin-Priority: 999
# STABLE deb http://ftp.us.debian.org/debian/ lenny main contrib non-free deb-src http://ftp.us.debian.org/debian/ lenny main contrib non-free deb http://security.debian.org/ lenny/updates main contrib non-free deb-src http://security.debian.org/ lenny/updates main contrib non-free # UNstable (sid) for gallery2 deb http://ftp.us.debian.org/debian/ sid main contrib non-free deb-src http://ftp.us.debian.org/debian/ sid main contrib non-free