The Leap 15 YaST2 /etc/sysconfig Editor on Leap 15.0 with Gnome installed as
Desktop and Plasma software pattern added, contains no option to switch between
GDM and typical SDDM:

Desktop > Display manager > DISPLAYMANAGER : gdm (option Xorg only, no sddm)
Desktop > Windows manager > Default_WM: gnome (no other options like Plasma5)


In comparision on Leap 42.3:

DISPLAYMANAGER : gdm (options: kdm, xdm, wdm, entrance, console, lightdm, sddm)
Default_WM: gnome (options: kde4, kde, lxde, xfce, twm, icewm etc)

This bug is related to Bug 1082713
https://bugzilla.suse.com/show_bug.cgi?id=1082713