A user-space linux driver for the Wii U GameCube Adapter.
./gcadapter_evdev
gcadapter_evdev will create up to four evdev devices dynamically, allowing usage in applications that do not explicitly support the Wii U GameCube Adapter.
You will need to have set the permission of the adapter before gcadapter_evdev will detect it.
Refer to https://wiki.dolphin-emu.org/index.php?title=How_to_use_the_Official_GameCube_Controller_Adapter_for_Wii_U_in_Dolphin#Linux for more details.
make
You will need libusb-1.0-0-dev and libgcadapter to build gcadapter-evdev.
Refer to the readme in libgcadapter/ before building.