Display manager

A display manager, sometimes known as login manager, presents the user with a graphical login screen to start an X server session.

Display managers in Portage

 * GNOME/gdm Display Manager
 * KDE Display Manager
 * LXDE Display Manager
 * Qingy A getty replacement
 * XDM X Display Manager
 * CDM The Console Display Manager
 * LightDM The lightweight display manager
 * SLiM Simple Login Manager
 * WINGs Display Manager

Configuration
Usually a display manager is started automatically on boot.

OpenRC
If you use OpenRC init system (default), the XDM init script handles the starting of the various display managers. You need to configure your default display manager, e.g. for KDM:

To start KDM on boot, add xdm to your default runlevel:

To start KDM now:

systemd
If you use systemd init system, first you have to get the file yourself. Some are in the systemd overlay. Pick one and move it to or install sys-apps/systemd-units from the overlay.

To start e.g. KDM on boot, enable the service:

To start KDM now:

Start X without Display Manager
See the X without Display Manager article.