I am also interested in this topic and cant find a straight forward answer anywhere! The emulators that I use are all RetroArch and I want to use my GPIO pins on my Pi2 as controls.
4x Joystick (up,down,left,right_
1x A
1x B
1x X
1x Y
1X L
1x R
1X Start
1x Select
1X ESC

I know how to find and edit the RetroArch.cfg file to change mappings for a keyboard but I was wondering if anyone knew how to map GPIO pins instead of keys?
I’m beginning to think buying a keyboard encoder/ I-PAC is the easiest way of doing this