When I upgraded to Debian 5.0 lately I noticed that there is no Gallery2 package anymore in Lenny. So I asked the Debian maintainer about this.
Here’s what he replied:
Gallery 2 is not available in Lenny due to large amount of differences between the previous version and 2.3-1 from unstable. The release team and I decided not to include Gallery 2 in stable due to this disruptive change late in the release process. The gallery2 package from sid can be installed via pinning.