The GC adapter for Wii/WiiU works just how it sounds like. It's an adapter that plugs in via WiiMote slot and allows you to use the GC controller instead of Wii classic controller. It even has a turbo button. The only issue is the compatibility scale. While it does say Wii U support, I'm not sure how far. The fact that you're plugging it the WiiMote suggests that it only accepts input for games that support the Wii Classic Pro controller and not necessarily the Wii U pro controller. It also may not support vibration functionality but that's not a major deal breaker for me.
You can order your copy here if you're interested--it's the cheapest one I found that's internationally friendly. If it does work, that's good. Whatever happens I hope that Nintendo is watching this so they may hopefully release an official adapter, or better yet, a fully compatible wireless GC controller for the Wii U.