Update: As several helpful readers have pointed out, this hack is actually for the Wii U Pro Controller and not the GamePad — although someone is working on it. Apologies for any misunderstanding or disappointment caused.
Those bedroom coders. Is there anything they can't do? Take the Wii U GamePad, for example — there's now a Windows-based application which allows you to use the GamePad to control your PC.
It's not an entirely straightforward process and you may need to tinker around with the Bluetooth drivers on your computer, but it does work.
Another catch is that currently it's only possible to map keyboard keys to the various inputs on the controller — so the analogue sticks won't work properly. However, where there's a will, there's a way, and another coder is working on an app which makes the PC think the Wii U GamePad is an Xbox 360 controller, which should ensure analogue support.
Is this something you're personally interested in, or will you be keeping your GamePad exclusively for its intended purpose — interacting with the Wii U itself?