Unlike the first five years of Wii consoles, the Wii U does not play GameCube Game Discs. One workaround is to buy both a Wii U and a GameCube, mod the GameCube for HDMI, and play Wii games in the Wii U. There exists a
GCVideo internal mod to add HDMI to a GameCube, available as a DIY kit or as an installation service. But I'm not aware of any source for pre-modded GameCube consoles, and the external mod for the DOL-001 is waiting on manufacture of a plug to mate with the proprietary Digital AV Out receptacle.