One thing that tripped me up until FenrirWolf set me on the right path:
https://wiki.dolphin-emu.org/index.php?title=How_to_use_the_Official_GameCube_Controller_Adapter_for_Wii_U_in_Dolphin
As he also noted, the link may not work to the page (it came up as a blank wiki entry for me). You may need to go to the wiki and search for 'official gamecube adapter' to find the setup.
The Zadig driver worked great for me. I ordered a second GC pad (one of the real Nintendo Japanese language Super Smash Brothers-labeled ones) and received it a couple days ago. Plugged it into port 2 and it was recognized by Dolphin with no tweaks needed from me!
https://wiki.dolphin-emu.org/index.php?title=How_to_use_the_Official_GameCube_Controller_Adapter_for_Wii_U_in_Dolphin
As he also noted, the link may not work to the page (it came up as a blank wiki entry for me). You may need to go to the wiki and search for 'official gamecube adapter' to find the setup.
The Zadig driver worked great for me. I ordered a second GC pad (one of the real Nintendo Japanese language Super Smash Brothers-labeled ones) and received it a couple days ago. Plugged it into port 2 and it was recognized by Dolphin with no tweaks needed from me!