The GC+ is a great bit of hardware, but using it through an adapter is definitely a bit of a backwards way to do it for something like this. I'm happy to share my code, you don't even need a custom PCB as you can buy an arduino board off the shelf that will do exactly what you need. Here's the board I recommend: https://store.arduino.cc/usa/arduino-microJust started looking into pi portables myself and definitely will be watching this thread. I too am interested in your controller code and how you implement the controller altogether! I was just going to experiment using a GC+ with a gc to usb adapter for a pc and hope that works with the pi... Looking forward to seeing more!
There are other cheaper options too, there seems to be plenty of "Pro micro" clones around for just a few dollars. The only downside to those is they have a few less pins, so you can't quite do a full controller with dual analog sticks, dpad and 8 buttons. If you are happy with that as a limitation you can go that way too.
If you decide to go down this path, let me know. I'll draw up a little diagram to show you where and how to connect your buttons to the board and I'll send over a copy of my code.