SDDM (Simple Desktop Display Manager) is the recommended login manager and is pulled in automatically via kde-plasma/plasma-meta by default. This is the preferred option. Alternatively, LightDM can be used and pulled in by setting USE flag
-sddm for kde-plasma/plasma-meta. Change the setting accordingly in /etc/conf.d/xdm. Also, be sure to read through the SDDM page if further issues appear.