In Ubuntu 20.04.1 the latest gdm3 package is only 3.36.3-0ubuntu0.20.04.1 and I have a similar, although probably different problem:

GDM wouldn't show a login screen on the second seat connected to an NVIDIA graphics card. After disabling Wayland in /etc/gdm3/custom.conf the problem disappeared and I again got a login screen on the second seat.

I would suspect that maybe the NVIDIA blacklist is part of the problem because gdm3 decides to start a Wayland session for seat0 and an X11 session for seat-1. Why can't it start a Wayland session for both seats?

My hardware:
00:02.0 VGA compatible controller: Intel Corporation Xeon E3-1200 v3/4th Gen Core Processor Integrated Graphics Controller (rev 06)
01:00.0 VGA compatible controller: NVIDIA Corporation GF119 [GeForce GT 610] (rev a1)

Intel is seat0 and NVIDIA is seat-1

Attached please find the gdm3 debug log, the interesting parts about seats look like this:

$ grep seat gdm-multiseat-wayland-nvidia-problem.txt 
Okt 28 09:29:08 brho gdm3[8608]: GdmLocalDisplayFactory: enumerating seats from logind
Okt 28 09:29:08 brho gdm3[8608]: GdmLocalDisplayFactory: wayland login display for seat seat0 requested
Okt 28 09:29:08 brho gdm3[8608]: GdmLocalDisplayFactory: Adding display on seat seat0
Okt 28 09:29:08 brho gdm3[8608]: GdmDisplay: seat id: (null)
Okt 28 09:29:08 brho gdm3[8608]: GdmDisplay: seat id: seat0
Okt 28 09:29:08 brho gdm3[8608]: GdmSession: Created user session for user 129 on display /org/gnome/DisplayManager/Displays/94424245309920 (seat seat0)
Okt 28 09:29:08 brho gdm3[8608]: GdmLocalDisplayFactory: X11 login display for seat seat-1 requested
Okt 28 09:29:08 brho gdm3[8608]: GdmLocalDisplayFactory: Adding display on seat seat-1
Okt 28 09:29:08 brho gdm3[8608]: GdmDisplay: seat id: (null)
Okt 28 09:29:08 brho gdm3[8608]: GdmDisplay: seat id: seat-1
Okt 28 09:29:08 brho gdm3[8608]: GdmServer: Starting X server process: /usr/bin/Xorg :0 -background none -noreset -verbose 7 -logverbose 7 -core -auth /var/run/gdm3/auth-for-gdm-P9W4S0/database -seat seat-1
Okt 28 09:29:08 brho gdm3[8608]: GdmSession: type wayland, program? yes, seat seat0
Okt 28 09:29:08 brho gdm3[8608]: GdmSession: type wayland, program? yes, seat seat0
Okt 28 09:29:08 brho gdm3[8608]: GdmSession: type wayland, program? yes, seat seat0
Okt 28 09:29:09 brho gdm3[8608]: GdmLocalDisplayFactory: creating new display on seat0 because of VT change
Okt 28 09:29:09 brho gdm3[8608]: GdmLocalDisplayFactory: wayland login display for seat seat0 requested
Okt 28 09:29:09 brho gdm3[8608]: GdmSession: Created user session for user 129 on display /org/gnome/DisplayManager/Displays/94424245244496 (seat seat-1)
Okt 28 09:29:09 brho gdm3[8608]: GdmSession: type (null), program? yes, seat seat-1
Okt 28 09:29:09 brho gdm3[8608]: GdmSession: type (null), program? yes, seat seat-1
Okt 28 09:29:09 brho gdm3[8608]: GdmSession: type (null), program? yes, seat seat-1
Okt 28 09:29:10 brho gdm3[8608]: GdmLocalDisplayFactory: X11 login display for seat seat-1 requested


For me the workaround was to disable Wayland complete like this:

$ cat /etc/gdm3/custom.conf 
[daemon]
WaylandEnable=false

It would be very nice to get multiseat to work again and please also with Wayland (X is dead, right?)