Lost Gnome after installing flakes and home-manager

Hi. I installed the full graphical NixOS + Gnome from the latest installer this morning. Afterward, I just added flakes and nix-command features to the configuration.nix and then copy pasted my somewhat minimal flake.nix and home.nix over from another machine.
After running sudo nixos-rebuild switch --flake .#newMachine, the system went from being a pretty Gnome UI to the minimal “Welcome to NixOS” prompt.

Is that to be expected?
Once flakes or home-manager are installed, does the displayManager/desktopManager stuff need to be configured there instead?
I considered the configuration.nix where system level services are defined so I assumed that would persist when moving to using a flake that still pulls in the configuration.nix as a module.

I feel like I should add on now that I’ve done some debugging.
sudo journalctl -b -u display-manager logs out a bunch of errors now that the display manager isn’t working correctly.
Things like

systemd[1]: Starting Display Manager...
lightdm[2092]: Error updating user /org/freedesktop/Accounts/User1000: GDBus.Error:org.freedesktop.Accounts.Error.PermissionDenied: Not a>
display-manager[2092]: Failed to use bus name org.freedesktop.DisplayManager, do you have appropriate permissions?
systemd[1]: display-manager.service: Main process exited, code=exited, status=1/FAILURE
systemd[1]: display-manager.service: Killing process 2100 (X) with signal SIGKILL.
systemd[1]: display-manager.service: Killing process 2112 (rm) with signal SIGKILL.
systemd[1]: display-manager.service: Failed with result 'exit-code'.
systemd[1]: Failed to start Display Manager.
systemd[1]: display-manager.service: Triggering OnFailure= dependencies.
systemd[1]: display-manager.service: Failed to enqueue OnFailure= job, ignoring: Unit plymouth-quit.service not found.
systemd[1]: display-manager.service: Scheduled restart job, restart counter is at 3.
systemd[1]: Starting Display Manager...
systemd[1]: Started Display Manager.
lightdm[2185]: Error updating user /org/freedesktop/Accounts/User1000: GDBus.Error:org.freedesktop.Accounts.Error.PermissionDenied: Not a>
systemd[1]: display-manager.service: Deactivated successfully.

And if I comment out the gdm and gnome related lines in my configuration.nix, I do get a pretty login screen again but I can’t login because it doesn’t have a desktop manager.

I think I figured out the issue. Now I’m working on the solution.
I have two graphics cards (both NVidia) on this computer and (I guess) the gdm display manager was trying to use the other card that wasn’t hooked up to a monitor or just getting confused with what to do with two cards.