You'll have to explain what you have so far, technically.
If you're just starting out with an idea and some pages of doodles, you have a fairly long road ahead of yourself. If you have something, but it's done in a proprietary "$GENRE maker" environment, then you also have a fairly long road, although slightly shorter.
The ideal place for you to be to start considering a GP2X port would be to have a finished game in SDL.
But yes, we'll need to know more about your technical progress before we can advise a regimen for porting/development.