You don't put GC Discs in your Wii U, you have to validate the games on an actual Wii first whilst using a WiiMote, it stores the validatioin on the WiiMote and then you can use the .iso on your WiiU.
- Insert a GC game into your Wii (Not WiiU)
- Use CleanRip to rip the game to USB or SD Card depending where you want it.
- Put the USB/SD Card into a Computer or use Wiixplorer and move the .iso you just created into the /games folder on your USB or SD.
- Rename the .iso from it's game ID to something you remember, eg GPEOD.iso into GameName.iso
- Eject that disc from Wii.
- Put the USB/SD into the Wii and boot into HBC.
- Launch devolution and press either 1 for SD or 2 for USB on the WiiMote.
- Choose your game and press A on WiiMote.
- Wait until the Wii flashes blue twice, over and over.
- Make sure the WiiMote is on again and insert your game you just ripped into the drive.
- It will validate it on the WiiMote and launch it from the SD/USB eventually.
You can now just use that stuff on WiiU without the GC Disc because it's verified.
- Boot devolution on WiiU.
- Press 1 for SD or 2 for USB.
- Select your game and press A.
- Make sure WiiMote you verified with is active (Press buttons till it connects).
- Game will boot.