Usually in Gnome and KDE it is only possible to define shortcuts with the Super key in combination with another key. But is it possible to use only the Super key as a shortcut, and how could I achieve this?
Yes, you can. You can fill with Super_L to the shortcut you want, for example to use win key only for launching gnome menu :
open terminal, type :
The gnome menu will lauch with windows key only.
The Super key is a so-called modifier key, i.e., it is only meaningful in combination with some other key (it "modifies" the other key to report Super+key instead of just key). This makes it impossible to bind it to a shortcut with the System->Preferences->Keyboard shortcuts GUI without a tweak.
You can tell the system to not consider Super a modifier key any longer; then you will be able to map it to any shortcut you want.
Note: These settings will not survive a reboot; you can make them permanent
by creating a text file