There has been the libdrc.org open-source project for supporting the Wii U gamepad under Linux but a proper kernel driver with mainline ambitions has been in the works more recently with the latest code having been volleyed today.
This "wiiu-drc" Linux driver is for supporting the Wii U gamepad when connected via wireless to the DRH of the console (details on the Wii U gamepad hardware architecture here ). The DRH is an internal chip that in turn exposes it as a USB device. The kernel driver exposes it as a conventional game controller with the buttons and sticks then working under Linux.
The Wii U GamePad also has a touchscreen display as well which this driver should support. This Wii U DRC kernel driver was initially published earlier this month while the latest spin of it can be found via this patch series . In addition to the basic controller support, there is also working battery reporting, accelerometer, gyroscope, and magnetometer readings with this wiiu-drc driver.
This driver should be useful for those wanting to get Linux running on the Wii U game console. The Nintendo Wii U console still seems available in some refurbished/used sales channels for $230~280 USD for those looking to experiment with Linux on it, granted the hardware specs aren't all that exciting in 2021 with a PowerPC 750 tri-core processor, 2GB of RAM, etc, but at least has a fairly interesting gamepad.