A Wii U GamePad update is required.

[nastys]

The update will always fail to install. It is hard to bypass this message.
Maybe the region is hardcoded?

[rolandoislas]

What version/branch is this and what is the region of the Wii U?

[nastys]

I was using the master branch but I've noticed that the development branch is able to bypass that, somehow.

After clicking OK I can use the console normally. I don't know if it is intended to do that but it works.
By the way, Wii U is European.

[rolandoislas]

The Wii U sends a command with the service id 5 and method 0x06. The response it expects is the firmware version.

From the libdrc documentation:

Returns the 4 byte UIC firmware version followed by the first 768 bytes of the UIC EEPROM.

At the moment the dev branch sends null data and the master/version 1.1 send an old European version. With the correct, dumped version data the gamepad would register as updated to the Wii U.