It would not have been included with the main DNN installation as it would not have passed the new review processes yet (not up to scratch if you like).

There are quite a few outstanding issues in gemini for this project:
http://support.dotnetnuke.com/project/Project.aspx?PROJID=6

Best way to test the gallery module is to try is with a test installation of 4.5.1. Let us know your findings!

Note if you need a good third party module, the one from Ventrian is excellent:
http://www.ventrian.com/Products/Modules/SimpleGallery.aspx

By the look of it the same team who do the forums module also does the gallery module. It looks like the team are spending all their time building an excellent forums module right now. It would be nice if the team could comment on the status of the project however, just as an update. Even if nothing is happening right now, we know you can't do everything all at once.

Many thanks....

I use the Polle PiGa gallery when I because frustrated at the DNN gallery (sorry- not very positive feedback I know). Greate tool and way ahead of the DNN one.