I asked about 18 months ago if gallery2 was available in any of the CentOS repositories, and it seemed that at that time it was not. However, there was some talk of putting it in epel or epel-testing. But I looked just now, and did not find it there.
I actually installed gallery2 on my server by following the instructions at http://codex.gallery2.org/Gallery2:Preinstaller. But I've had a slight http problem on the server which might be related to gallery2 (wireshark seemed to show some odd calls on gallery2); so I was wondering if there was yet an official CentOS implementation?