SDDM

Simple Desktop Display Manager (SDDM) is a display manager that supports X11 and Wayland.

Emerge
Install :

If the are performance issues, it might help to add the sddm user to the video group:

Configuration
The configuration file for SDDM is:

OpenRC
Set SDDM as the default display manager:

To start SDDM on boot, add dbus and xdm to the default runlevel:

To start SDDM now:

After logging in to the X session via sddm, it is a good idea to verify that ConsoleKit is working as intended. By typing all active sessions can be listed. The list should include your session, typically running on. This session should also read. Active being FALSE indicates an issue.

systemd
To start SDDM on boot:

To start SDDM now:

Plasma
Graphical configuration is integrated in Plasma 5 system settings by installing :

Without systemd
Add xdm to the default runlevel and set:

After logging in to the X session via sddm, it is a good idea to verify that ConsoleKit is working as intended. By typing all active sessions can be listed. The list should include your session, typically running on. This session should also read. Active being FALSE indicates an issue.

Permission denied errors in Xorg.log
The X server will not start and permission denied errors (such as the following) are present in the log file:

It is likely the sddm user has not been added to the video group. Running the following command should fix the problem: