So I believe I have a Mayflash USB Gamecube > PC USB adapter - lsusb says it's: Bus 001 Device 016: ID 1a34:f705. It looks just like the one in the picture on the manufacturers' website here.
As for the problem, it seems like it's not supported - nothing happens when I check it using a game development library, or in other games. By the way, a method to test connected gamepad devices would be nice (I just opened a request for jstest-gtk).
I'd like to request support for this controller / USB device, if possible.
Running Solus fully updated - 1.2.1.