'gnome-desktop-environment' is an obsolete metapackage, now replaced with 'gnome'. It installs the standard upstream GNOME collection of software (close to what you find on Debian). If i try to install this package, apt wants me to install lots of applications and themes - 73 packages which would use 261 MB of disk space. And I'm using a very comfortable GNOME session already.
Obviously Ubuntu's set of default applications is different, so the download will be big.
gnome-shell's dependencies alone include gnome-panel, metacity, panel applets and other stuff that are related to the fallback mode. Naturally, none of this is installed on a default Ubuntu system.
None of this changes the fact that Unity simply sits on a GNOME 3 environment. It's just the shell.