Adicione isso a /etc/yum.repos.d/public-yum-ol6.repo
[ol6_gdm_multiseat]
name=Oracle Linux 6 GDM Multiseat ($basearch)
baseurl=http://public-yum.oracle.com/repo/OracleLinux/OL6/gdm_multiseat/$basearch/
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-oracle
gpgcheck=1
enabled=1
Então você pode instalar usando yum install gdm-multiseat