Patched version of Nintendont for the Wii U's vWii to allow for native gamecube controller input. No gamecube controller adapter needed. Requires hardmod. Navigate to settings and enable "Native Control" - It's greyed out but this patch lets you tick the option. Just replace /apps/Nintendont/boot.dol with the one provided below.
https://bitbuilt.net/forums/threads/gc-controller-natively-on-the-wii-u.6441/
Insane work by Lazr for finding the controller data pin.