GC/Xbox pad to usb converter?
Dunno about GC, but Xbox pads are simply usb devices anyway. Get an old USB cable (or visit Maplins (electronics supply shop) and buy the connector for just under a quid) and hook the coloured cables up like for like (specs for hooking up just the connector easily available) with the exception of the Yellow one, and get the driver off the net.
The best thing about the Xbox one is you don't have to wreck a controller to do it - just use the stubby breakout cable.
Dunno whether the official pads work (I expect they would), but I did this for the living room Mame box with some Gamester Arcade Sticks. Worked a treat, but Logic3's Arcade Stick didn't (good thing too - its about the size of a PS2!)