like he said wiiu gc adapter only sends controller signals so it doesnt send anything the gba link cable does so yeah no connection can be made on wiiu.More specifically, the GCN Controller Adapter only supports HID (for controllers) and not the GBA link protocol. Nothing's impossible about what was asked, but you would need to implement support in Nintendont, and have the hardware you'd need to connect a GBA to a USB port.
Incredibly esoteric when you could just buy a Wii. And a Gamecube controller, for the time being. They're pretty cheap, and so are the component cables for them. (Not like the Gamecube!)