I agree with junebug172... there's something wrong if you can't access your own private galleries on your own AppleTV. I have a gallery that I created to use as a screensaver, but I don't want to make it public. People in my house seeing the pictures on the TV is OK... people I don't know seeing the pictures, not OK. This seems to be an oversight in the 2nd generation Apple TV.
As an attempted workaround, I made the gallery public, selected it on the AppleTV, then reset the gallery to private (hoping it would remember where the pictures were). It didn't work.
This really needs to be fixed.