Yeah, ChromeOS uses GNU coreutils and glibc; it's a Gentoo derivative, not some alien like Android. I don't know how you define "usable", but if you switch to developer mode to let you get a shell at all then it feels about like any other GNU/Linux distro.
>it's a Gentoo derivative, not some alien like Android
Android is no more of an alien than Gentoo.
>I don't know how you define "usable"
It means when the user logs in they are given a bash shell so that they can use GNU coreutils. This is similar to UNIX in which you log in and are then able to use UNIX's built in utilities.
Personally, it doesn't qualify as GNU to me as the only GNU software included AFAIK is Bash. To me GNU requires coreutils to be there and usable.