Cannot change GDM default session system wide
Feature summary
It should be possible to change the system wide default session for new users in the GDM custom.conf config file. In the past there was the DefaultSession options, but it looks like it's not working any longer. For example see also the Fedora documentation mentioning DefaultSession at the bottom: https://docs.fedoraproject.org/en-US/quick-docs/configuring-xorg-as-default-gnome-session/
How would you like it to work
Change a setting in /etc/gdm/custom.conf to select a default session for all new users, so users not having one selected already via the account service (which should remember the last one used as it's already doing right now).
Relevant links, screenshots, screencasts etc.
This can be a common scenario both in a personal / family or enterprise environment. It might be desired to have the default session to not be GNOME by default for all new users. My family (the most basic users you can imagine without any computer knowledge) finds KDE Plasma more intuitive, so I would like them to use Plasma by default. I prefer to use GDM as the default login / display manager as it's the only one allowing screen locking in GNOME, while retaining full compatibility, to my best knowledge, with all other desktop environment.
When logging in the first time using GDM the default session is always GNOME. For my parents it's hard to understand they have to go to the little gear in the bottom right of the screen and select Plasma. If they login into GNOME by accident they are also lost and don't know how to logout, as they are not familiar with it.
This has been a fairly important source of problems when wanting something other than GNOME as default. It happens only at the first login with a new computer or when a computer is reinstalled from scratch, as once they login with Plasma the Account Service will remember the settings, however it's still a fairly major feature missing from GDM in my humble opinion.
Thank you. Kind regards.