Link to the Zadig Driver installer: https://github.com/pbatard/libwdi/releases

Download and launch Zadig. If you're using the a third party adapter, like the one from Mayflash, make sure you switch it to "Wii U" or "Switch" mode or Zadig won't pick it up properly. If your adapter has a Slippi mode, use that.

From the "Options" menu in Zadig, select "List All Devices"
In the pull-down menu, select "WUP-028". Ensure that its USB ID is "057E 0337".
*If it does not appear in the list then try inserting the adapter (specifically its black USB cord) into another USB port and make sure it's in Switch/Wii U mode.
On the right column, select "WinUSB" then click "Replace Driver". Select "Yes" to modify the system driver.

When it says "the driver is installed successfully", then close Zadig and go enjoy peak! You may (though probably not) need to restart your computer after installation.


Link to the original, unmodified Smash Wiki article for dolphin in case I borked something:
https://wiki.dolphin-emu.org/index.php?title=How_to_use_the_Official_GameCube_Controller_Adapter_for_Wii_U_in_Dolphin

If you have a Mayflash GC adapter with PC mode, but you notice issues like analog stick movements not controlling your character properly, or L and R buttons having weird behaviors like getting stuck or doing repeated presses, use this Steam controller layout. With Steam running, just paste the text below into your browser's URL bar and confirm the "open Steam" dialog box if it appears.
steam://controllerconfig/2217000/3404984290
(Or, go to controller settings from either your Steam library page for RoA2 or the home button menu while the game is running, and look for "FIXED Mayflash GCN Adapter in PC mode v1.1" in Community Layouts.)
This layout was made for the adapter model that says "GameCube Controller Adapter for Wii U/Switch/PC" on top, and does not have a Mayflash logo accompanying the text. It may not fix control issues present in other adapter models or ones not made by Mayflash.

If your adapter is only made for Wii U / Switch, follow these instructions courtesy of Reddit user SwarmsofJelly thoroughly, fully copypasted here:
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Download the files from this Google Drive link https://drive.google.com/drive/folders/1jZ9wDjr6fXUMmwhYfIzJ_ExWjfehXnCc?usp=sharing

• This contains a already built version of ToadKing’s Wii U GC Adapter Driver (You can build your own from here https://github.com/ToadKing/wii-u-gc-adapter, make sure to install libudev, libusb, and libusb-dev/libusb-devel).
1) Most distros already have libusb installed but need the dev version to be recognized in the makefile.
2) Some distros (like Fedora) label the most up to date version of libusb as libusb1, so you need to get libusb-dev if that’s the case.
- You do not need these dependencies to run the built gc adapter driver provided above.,

Download the files, make sure you keep it in its own folder, and unzip anything that needs to be unzipped/extracted. The stuff about libusb and all that is already taken care of, so don't mind that.
Once you have the files, open the "wii-u-gc-adapter-master-built" folder with Kate. This is the same as right clicking it and hovering over "Open with" and selecting the Kate option. It should open up an unbelievably intimidating and overwhelming screen (at least for my first time). Ignore everything on that screen, look at the bottom and click "project." There should be a line that says something like "deck@steamdeck wii-u-gc-adapter-master-built)$" click on the line and type "make." Thats it, it installed the program. Theres a few more steps to make it work though.

You can follow step 2, but it really is optional. If you're having struggles putting the folder you create into the /etc/udev/rules.d/ folder, try opening the folder with kate and experimenting with a few things. I cant remember how exactly I did it, but I'm sure if you really need that official Dolphin support you'll be able to brute force this one.

Here are the rest of the steps before you can play!

1. In the Terminal/Konsole enter modprobe uinput
2. Plug in your adapter and set it to Wii U mode if it's 3rd party
3. In the terminal enter sudo, press space, and then drag the wii-u-gc-adapter file from the driver folder, afterwards enter you password and then the driver will start
steps 1-3 are actually 3-5 in the guide. The guide also talks about having to remap your controls, but YOU DO NOT NEED TO DO THIS (at least I didnt have to). Once the terminal/console talks about a port being recognized, you're good to play your games. If it doesn't read it still, try connecting and reconnecting your usb. From here you can exit out of desktop mode and go to the controller settings and remap your controls however you please from the built-in Steam controller settings.
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(M. Lozito has provided a mirror download link for the Linux files just in case the Google Drive link goes down.)
https://www.dropbox.com/scl/fo/ecuggi9r6hp24x6evh3gf/ACtelzYUP-XArKrDtdchTA4?rlkey=qy9jvyjjk7bcu1hpmkqwzld0l&st=yhsj25yk
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Finally, if you've exhausted all attempts to make your adapter work but can't, there are some you can buy that are newer and better-performing than ones made in the early Wii U/Switch years.

Confirmed working, recommend using XInput mode (green LED lights) for instant plug-and-play. Have to assemble it yourself, but worth it for the features and performance.
Handheld Legend GC Pocket Adapter[handheldlegend.com]

(Not tested - anyone who has, feel free to share in the comments)
Site includes files and instructions for Linux compatibility.
Input Integrity Lossless Adapter[www.input-integrity.com]

(Out of stock, not tested)
RaphNet adapter: Product page mentions Linux compatbility.
Single controller version[www.raphnet-tech.com]
Two-controller version[www.raphnet-tech.com]

Have fun!! Go to your local tournament!! ♥♥♥♥ Nintendo!!

If you need further assistance or if there's something I should add to the guide, let me know :)

Also give it a thumbs up if it helped you fix your stuff! I want free steam points yum yum yum yum