Use
Nintendont. It's designed to run Gamecube games in Wii mode; and this forwards compatibility comes with the advantage of being able to use Classic Controllers and Wii U Pro Controllers, along with many other options, such as emulated memory card images to save your games without a GCN card - as you can imagine, this is vital on Wii U. (
Devolution is also an option, but unless you plan to play LAN games like Double Dash or Air Ride, it's quite heavily deprecated. If you are, you can network with other Wii (U)'s running Devolution, actual Gamecubes with LAN adapters, and Dolphin.)
Just keep in mind that Gamecube USB adapters
do not support GBA connectivity, since they're purely HID devices, and lack the extra hardware the native ports in a GCN/Wii have for GBA link protocols. If you use
Nintendont in a Wii, you'll also want to use official cables, since Wii mode has stricter timings for GBA link hardware than GCN mode.