I want to use the GTD paradigm to quickly sort through my emails. How can I create these shortcuts: alt+shift+n to move to Now label, alt+shift+w to Waiting?
It doesn't look possible with the native Gmail interface.
While there is a "Custom Keyboard Shortcuts" Lab that you could activate, it doesn't let you define the actions you can take. All it allows you is to change the keys used to fire the already-defined actions.
It looks like you'll need a browser extension or some other tool to do exactly what you want.
However, maybe you can adjust to use the existing keyboard shortcuts.
V will open the "Move to label" dialog. Pressing a key (like N) will bring up the labels with that letter. (More letters will filter the labels if you have a lot.) Arrow to the one you want and press Enter. If you did something like use a special character only on your GTD labels that would make things quicker.