wiiu gc adapter or even switch gc adapter are not compatible with gba cable signals, nintendo made them only turn gc controller input into hid, while gba link cable does not send gc controller code therefore nothing comes out of there.
nintendont runs in wii mode therefore it only uses once core, you cant have a single core emulating gba games and then using it for
nintendont, it would never work it would be slow as hell even if it was possible to give the wiiugamepad a diferent stream than the tv, which in wii mode i dont think its possible tbh.