They are trying to become a channel just like Ubuntu and like every man and his dog seems to be doing lately. At this point their interests and those of the users don't necessarily align 100% of the time. The same applies to Google, Apple, Ubuntu, and a bunch of others really. If you just want the best possible desktop environment and nothing more, I'm sorry but that's not their priority now and compromises will continue to happen in that regard.
You can, theoretically, find apps outside of the "official" channels and install them. This is not what usually happens and this is not what's encouraged. The success and visibility of apps outside of these channels will always be limited at best, so de-facto they control what people run for the most part, and that's more than enough.
If you mean something different by "walled garden" then maybe we're discussing over semantics. Apple does go a step further though, if you mean that Apple's is the "only" walled garden.
In a walled garden, apps are taylored exclusively and you have to do it. They also impose conditions going further than licence and stability. Eventually they ask for a cut of anything you sell over there (which is something Debian hasn't ever done and won't do).
That's where they are going. Stay tuned.
Things that used not to belong in Gnome - it used to be a Desktop Environment with no more pretences than that.
They are taking steps in that direction.