Resposta retirada dos comentários dos OPs
% bl0ck_qu0te%root@saimon-System-Product-Name:~# dpkg --install /home/saimon/Downloads/gdm-2.20_2.20.10-0ubuntu3_i386.deb
(Reading database ... 165751 files and directories currently installed.)
Preparing to unpack .../gdm-2.20_2.20.10-0ubuntu3_i386.deb ...
Unpacking gdm-2.20 (2.20.10-0ubuntu3) ...
dpkg: error processing archive /home/saimon/Downloads/gdm-2.20_2.20.10-0ubuntu3_i386.deb (--install):
trying to overwrite '/usr/share/pixmaps/faces/astronaut.jpg', which is also in package gnome-control-center-shared-data 1:3.6.3-0ubuntu56.1
dpkg-deb: error: subprocess paste was killed by signal (Broken pipe)
Errors were encountered while processing:
/home/saimon/Downloads/gdm-2.20_2.20.10-0ubuntu3_i386.deb
Im realmente novo no linux im tentando configurar multiseat no meu PC e pilha aqui alguém pode me ajudar link