johnwasser:
If you want to emulate key presses with buttons it's cheaper to take the electronics from a used USB keyboard and connect buttons to pairs of wires to replace the keyboard keys you want.
^ This.
Or antoher one is an Leonardo, I haven't messed with one, but I think theres a library for them that allows it to emulate a USB keyboard or mouse perfectly, SO somethign like that will probably make your project a bit easier (some USB keyboards can suck pretty hard to take apart)
I will note though, GBA will require 10 buttons (Up, Down, Left, Right, A, B, R, L, Start, and Select)