In most of my internet searching on this issue, I came across very little helpful information. This is my experience in trying to get a gamecube controller working with Unity. If you have a different experience or need help figuring something out, feel free to reach out to me at my email
Anyway, lets get on with it.
To start, you need to have an adapter plug your gamecube controller into your computer. If you’re using some weird USB Gamecube Controller I don’t know how to help you, I’m sorry. If you have an unofficial adapter (like me), you will have to follow these instructions, if you have an official one it might work out of the box but I’m unsure. I’d recommend following that link and installing Zadig to configure your controller to begin. In addition, if your Adapter has a toggle for “PC” and “Wii U”, it must be set to “PC” in order to work with Unity, so if you are not getting any input from your controller that is a good thing to check.