If you can't start the X environment, you cannot run the Add/Remove software application (
pirut from the command line). In text mode you have to deal with
yum, the package manager. Indeed Add/Remove Software is a wrapper to run yum commands in a more comfortable graphical interface. Here is the documentation from the CentOS site:
http://www.centos.org/docs/5/html/yum/.
If you want to re-install (or add missing component to GNOME) you can try to install the whole group of packages. Here is some useful command line:
Code:
yum grouplist ### lists all installed and available groups
yum groupinfo "GNOME Desktop Environment" ### lists the packages in the group classified
### as mandatory, default and optional
yum groupinstall "GNOME Desktop Environment" ### installs the specified group