Nintendont -, Yes, you CAN play GameCube games on your Wii U!

Like all homebrew and modification, INSTALL AT YOUR OWN RISK. Nintendo so far have shown they will not ban any users for hacking Wii mode on the Wii U, but it may void your warranty. This is purely a modification of the Wii Mode on Wii U and will not touch or affect anything on Wii U mode at all

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Nintendont - Yes, you CAN play GameCube games on your Wii U

ConsoleTuner » t2:usage_guides:systems:wii

So Nintendont was updated to support the GC adapter for Wii U

Super Smash Bros. for Wii U: Controllers