'libreoffice-gnome' is a non-existent source package, while there's a binary package called 'libreoffice-gnome', which is built from the 'libreoffice' source package. IMHO, to prevent user confusion, 'libreoffice-gnome' src package should somehow be removed from Debian BTS: http://bugs.debian.org/cgi-bin/pkgreport.cgi?src=libreoffice-gnome