Whonix 15 was using lightdm. It was removed due to autologin heisenbug. [1] [2] [3]
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 [4]).
[1] autologin not work sometimes · Issue #210 · canonical/lightdm · GitHub
[2] autologin is not working every time · Issue #191 · canonical/lightdm · GitHub