Okay, I've been trying this for two days and I'm at lost.
I used to have xdm managing my login screen. But I like the way ( eye candiness ) from gdm so I wanted to change it.
I modified the file, /etc/X11/default-display-manager. When I checked both gdm and xdm configs, it says that they confirm to default-display-manager file and the settings were all correct. xdm was in /usr/bin/X11/xdm and gdm was in /usr/bin/gdm .
When I booted up, it says gdm is not configured as default display manager and xdm is not configured as default display manager. Where is that setting should go in order to make gdm the default display manager? I've looked in startx, xinitrc file.
Distro is Debian Unstable.