SDDM

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

Emerge
Install :

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

Files
The global configuration file path for SDDM is. See comments in the file and for details on available options.

Keymap
To select correct keymap on login screen, add following line to the file:

OpenRC
Set SDDM as the default display manager:

To start SDDM on boot, add 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 :

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:

Missing users
If the login screen is missing some user(s) to choose from, this might be caused by sddm.conf - standard MinimumUid is 1000 and some existing users may have lower Uids.