For years now Google has used Ubuntu as the base for their own private in-house OS, Goobuntu, to what they are now calling gLinux, which will be based off the unstable Debian-Testing branch for Debian 10 “Buster.”
Using the testing branch of Debian does mean that the stability of the OS could be questioned, since Debian Stable is known as virtual unshakable, but Debian testing is almost the complete opposite at times.
During the announcement which was done at Debconf2017, Margarita Manterola, an engineer with Google, explained how packages will be rebuilt, and then tested. If a package is buggy or needs work, it will be patched and worked on by Google, before it is then inserted into the gLinux operating system.
With that said, gLinux again is a private and in-house OS used by Google, and is not something that will likely ever be released to the public.
Martin's note: It appears, however, that Google plans to contribute to the Debian Project which will benefit the community.
Google has not made public what desktop environment will be used, leaving some to guess or theorize. I personally believe that either GNOME or Cinnamon will be used, given that GNOME is once again growing in popularity, and is very customizable.
However, I also wouldn’t be surprised if KDE could potentially be used, given its very advanced and already powerful tools such as the amazing File Manager, Dolphin.