The Wii U GameCube Controller adapter is compatible with all games that supported any variant of the Wii Classic or Pro controllers.
Update 2: Nintendo responded to our request, and stated that the original listing on the UK store was indeed a mistake. It clarified: "Please note the GameCube Controller Adapter for Wii U is compatible with Wii U and Super Smash Bros. for Wii U specifically - it is not compatible with any other Wii U software. The adapter is also not compatible with Wii and we apologize for any confusion prompted by our earlier listing"
Update: The listing for the GameCube Controller adapter on the website has been changed, and no longer includes the line that states it is compatible with Wii Classic Controller, Wii Classic Controller Pro, and Wii U Pro Controller titles. We have reached out to Nintendo for clarification.
Original Story: A listing for the Wii U GameCube Controller adapter on Nintendo UK's official website has stated that it will support a wide variety of Wii U titles. According to the listing, any game that supported either the Wii Classic Controller, Wii Classic Controller Pro, or Wii U Pro Controller will be compatible with the GameCube Controller adapter.
This is great news, as it was previously believed that the adapter would only function with the upcoming Super Smash Bros. for Wii U, but supporting games that use the Classic and Pro controllers means that the vast majority of Wii and Wii U titles will be supported.
The GameCube Controller adapter will be released alongside Super Smash Bros., as a stand-alone purchase, or in a bundle with a copy of the game and an additional GameCube controller. It plugs into the two USB ports on the front of the Wii U, and allows up to four GameCube controllers to be used with the system.
Nintendo is also releasing some new Super Smash Bros. themed controllers to go with the new title.
Curiously, the listing also states that the adapter works with the older Wii console, which already contains ports for four GameCube controllers.
Source: Nintendo UK