Nintendo implemented a region-lock on the Wii U console. As a result, titles purchased in North America won’t work on a system in other countries (and the same goes for other territories).
The region-lock seems to goes beyond this. It appears that GamePad controllers are also locked to their country of origin.
News floating around Japanese blogs indicates that a GamePad purchased overseas won’t work on a Japanese Wii U. This is almost certainly the case for other territories as well.
It’s worth noting that all GamePads contain region codes in the model number. That should be the key when it comes to determining if a controller will work in a certain territory.