One thing GA will never be able to do is analyze the page views on the App Store and the source of these page views. I think that will be the great strength of this tool. Download spikes after a press article/review won't remain unexplained anymore.
So does Google have a similar policy where only GA can get information about users in the Play store, or is that not tracked at all?
Well it's not really a question of policy, but there is no way to inject analytics on the Play Store other than link it with GA. And there is no way to have analytics on the App Store other than iTunesConnect Analytics.