Trisquel 7.0: "gnome-shell" can't be installed - depends on virtual package "gir1.2-gdm-1.0".

As of 19 July, 2014:

$ sudo apt-get install gnome-shell
[...]
The following packages have unmet dependencies:
 gnome-shell : Depends: gir1.2-gdm-1.0 but it is not installable
[...]

$ apt-cache show gir1.2-gdm-1.0
N: Can't select versions from package 'gir1.2-gdm-1.0' as it is purely virtual
[...]