The following post explains how to enable GUI root access on Debian 12. Instructions for Debian 11 are similar and can be found here. At this point I should warn you that using the root account is dangerous as you can ruin your whole system.  Try to follow this guide exactly.

1. First open a terminal and type su then your root password that you created when installing your Debian 12.
2. Install Leafpad text editor which allows you to edit text files (any other editor will do fine as well). Type: “apt-get install leafpad”
3. Stay in root terminal and type “leafpad /etc/gdm3/daemon.conf”. This command opens the file “daemon.conf” in leafpad. Under security type “AllowRoot=true”. So your security section in the file should look like this:[security]AllowRoot=trueOnce it looks like this save the file then exit the window.
4. Stay in root terminal and type “leafpad /etc/pam.d/gdm-password”. This command opens the file “gdm-password” in leafpad. Within this file you have comment out the line containing “auth required pam_succeed_if.so user != root quiet_success” so that it looks like this#auth required pam_succeed_if.so user != root quiet_successSave the file and exit.
5. Now you should be able to login as root in you GUI Debian 12.

In case you, you need Gui Root Login for older Debian versions:
Debian 8

Debian 9

Debian 9 KDE5 Plasma

Debian 10

Debian 11