OK, you have a PolicyKit problem: you just need to have an authentication agent running. That's why it works in KDE and not in GNOME. Start /usr/lib/policykit-gnome/polkit-gnome-manager in the background, and everything will be fine.
Now we need to find out why this daemon is not running automatically. Do you have a file at /usr/share/dbus-1/services/gnome-org.freedesktop.PolicyKit.AuthenticationAgent.service? Are there errors in /var/log/messages about DBus not being able to start that daemon?
OK, you have a PolicyKit problem: you just need to have an authentication agent running. That's why it works in KDE and not in GNOME. Start /usr/lib/ policykit- gnome/polkit- gnome-manager in the background, and everything will be fine.
Now we need to find out why this daemon is not running automatically. Do you have a file at /usr/share/ dbus-1/ services/ gnome-org. freedesktop. PolicyKit. AuthenticationA gent.service? Are there errors in /var/log/messages about DBus not being able to start that daemon?