SDDM

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

Emerge
Install :

Without systemd
If you see performance issues, it might help to add the sddm user to the video group.

Configuration
The configuration file for SDDM is:

Systemd boot service
To start SDDM on boot:

To start SDDM now:

OpenRC boot service
Set SDDM as the default display manager:

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

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.

As a fix you can try editing and removing the nox11 from the line mentioning :