Here is another observation (my old workaround):

- KMS is enabled, gdm starts in wayland mode. Trying to start gnome-shell in Xorg mode fails (gnome-shell/mutter crash).

- Switch to tty (ctrl+alt+f2), login and start:
- $ sudo lightdm --test-mode
 - lightdm is starting, nvidia logo appears for a moment
- Switch back to tty once again and kill lightdm (ctrl+c)
- Switch back to GDM (ctrl+alt+f1)
- Login in gnome-shell Xorg session: everything works fine at this moment.

The question is what lightdm does, that gdm doesn't?

New messages in logs during xorg startup after executing lightdm are:
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(0): Valid display device(s) on GPU-0 at PCI:3:0:0
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(0):     DFP-0 (boot)
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(0):     DFP-1
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(0):     DFP-2
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(0):     DFP-3
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(0):     DFP-4
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(0):     DFP-5
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(0):     DFP-6
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(0):     DFP-7


/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DELL U2412M (DFP-0): connected
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DELL U2412M (DFP-0): Internal TMDS
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DELL U2412M (DFP-0): 330.0 MHz maximum pixel clock
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0):
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-1: disconnected
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-1: Internal TMDS
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-1: 165.0 MHz maximum pixel clock
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0):
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-2: disconnected
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-2: Internal DisplayPort
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-2: 1440.0 MHz maximum pixel clock
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0):
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-3: disconnected
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-3: Internal TMDS
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-3: 165.0 MHz maximum pixel clock
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0):
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-4: disconnected
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-4: Internal DisplayPort
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-4: 1440.0 MHz maximum pixel clock
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0):
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-5: disconnected
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-5: Internal TMDS
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-5: 165.0 MHz maximum pixel clock
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0):
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-6: disconnected
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-6: Internal DisplayPort
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-6: 1440.0 MHz maximum pixel clock
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0):
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-7: disconnected
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-7: Internal TMDS
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0): DFP-7: 165.0 MHz maximum pixel clock
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(GPU-0):
/usr/lib/gdm3/gdm-x-session: (==) NVIDIA(0):
/usr/lib/gdm3/gdm-x-session: (==) NVIDIA(0): No modes were requested; the default mode "nvidia-auto-select"
/usr/lib/gdm3/gdm-x-session: (==) NVIDIA(0):     will be used as the requested mode.
/usr/lib/gdm3/gdm-x-session: (==) NVIDIA(0):
/usr/lib/gdm3/gdm-x-session: (II) NVIDIA(0): Validated MetaModes:
/usr/lib/gdm3/gdm-x-session: (II) NVIDIA(0):     "DFP-0:nvidia-auto-select"
/usr/lib/gdm3/gdm-x-session: (II) NVIDIA(0): Virtual screen size determined to be 1920 x 1200
/usr/lib/gdm3/gdm-x-session: (--) NVIDIA(0): DPI set to (93, 95); computed from "UseEdidDpi" X config