Hi Chengyi,
> In fact, gdm user doesn't need these services
I'm almost certain that this is incorrect. There's a good chance if you remove the systemd --user instance from GDM, you will end up with a broken/unbootable system.
Basically, instead of being its own service manager, new versions of gnome-session rely on the systemd --user instance. GDM needs gnome-session to be running correctly (to start GNOME shell, gnome-settings-daemon, etc in the greeter session).
The request to start a systemd --user instance comes from PAM. I'm pretty sure the GDM session uses the `gdm-launch-environment` rule. On Fedora 35, gdm-launch-environment pulls in system-auth, which then pulls in systemd. Your distro did not do this accidentally; GDM is explicitly configured to start the systemd --user instance (and any other service that PAM would start for a normal desktop session). This is because GDM is essentially just a normal desktop GNOME session with some customization applied.
I might have gotten a few minor details wrong here, because I'm not 100% familiar with the inner workings of GDM. But the main idea holds.
Is there any reason in particular you want to disable the systemd --user instance?
Regards,
Adrian
