Whonix 15 was using lightdm. It was removed due to autologin heisenbug.   
Therefore Whonix 16 is using gdm3. It is also a good choice because it supports wayland (and fits with Gnome, which Whonix might be ported to, depending on many things such as feasibility, resource use, please use a dedicated forum thread (none might exist yet, see ).
Yet another option might be not using any display manager and instead use startx (or its wayland equivalent).
For that purpose RAM Adjusted Desktop Starter - Kicksecure (rads) could be modified.
(Whonix is based on Kicksecure.)
If no display manager is installed and if there is enough RAM and if startx is available, start it.
How about Whonix-Host?
(Would equally apply to Kicksecure host.)
Pre-configured style: Should have display manager and no default autologin? Also for logon password at every boot required.
Minimal style: autologin using startx by default. Display manager installation, logon password setup should be left to the user?