At the moment, they are not releasing any technical information whatsoever, so developing a proper GPX2 emulator could be a problem, even with a gpx2 in front of you. Several people, including myself, have requested such information, and got either no reply at all, or a simple refusal.
However, they are saying that the SDK will be SDL based, so you'll be able to write your games and emu's on a PC or MAC, and then just recompile for the GPX2. No emulator needed. (It will however, be slightly slower than current GP32 methods of banging the hardware, simply due to the extra abstraction layer).